Quite often when trouble-shooting a Lightroom problem, it will be suggested that you 'reset' or 'trash' your Preferences file. There are few clearly identifiable situations that require this action, but as a 'last resort' it is often remarkably successful at fixing what are otherwise inexplicable problems.
Courtesy of Victoria, the extract from her book - The Missing FAQ - which explains how to do this reset is detailed below. However, before embarking on that action it would be useful to understand some implications:
Firstly, resetting the Preferences file will cause Lightroom to create a new one when it is next started, and this new file will have all settings at their default positions. If the act of resetting the preferences file fixes the initial problem then obviously you will want to keep the new file. However, any changes which you had previously made to the Lightroom Preferences will have been undone. In itself, this is a simple thing to correct: you could for example take a screenshot of each of the tabs in the Preferences settings before the reset (5 or 6 depending upon Lightroom version), then a simple comparison after the reset will enable you to restore the settings to the way you had them. Similarly, any changes you have made to the various Grid and Loupe View Options will also be reset to defaults, so again a screenshot of the View Option tabs (Ctrl+J on Windows, Cmd+J on a Mac) will help in restoring things back to the way you had them.
A more pressing issue, however, concerns the fact that Lightroom's knowledge of your catalog(s) will have been lost during the reset, and the 'Default Catalog' setting on the Preferences>General Tab will be set to 'Load Most Recent Catalog'. Therefore (and this depends entirely on how you have chosen to start Lightroom, and where your catalog is normally stored), you may run into an unexpected situation when starting Lightroom after you have reset the Preferences file. There are several different scenarios, for example Lightroom could start with a new blank catalog, or it could start with a pre-existing older catalog, or it could find a catalog from a previous version and so ask if you want to upgrade, or it could find your 'proper catalog'.
To circumvent such unexpected scenarios, I would suggest the following prior to resetting the Preferences file: with your normal catalog open in Lightroom, open the General Tab of the Catalog Settings dialog box (under Edit menu on Windows, Lightroom menu on a Mac) and make a note of the file name and location of the catalog (the full path is listed there).
Then after you have closed Lightroom and reset the Preferences file as per Victoria's instructions below, use Windows Explorer (on Windows) or Finder (on a Mac) to locate the catalog file that you noted above, then double-click on the *.lrcat catalog file. This will launch Lightroom with your correct catalog open, thus there should be no possibility of opening an incorrect catalog by mistake.
After Lightroom has started, you can then test to see if the reset of the Preferences file has fixed the initial problem. If it has not, then you can simply revert to the original Preferences file by deleting the newly created Preferences file and reversing the reset action (rename is my preferred action, BTW).
However, if the action HAS fixed the problem you will obviously want to keep the new Preferences file.....in which case you should spend a few minutes going through the various tabs and ensuring that you get all the settings the way you had them. Pay particular attention to making sure that the Default Catalog setting on the General Tab is correct.
NOTE: Obviously the above instructions assume that you are able to start Lightroom normally in order to examine existing settings. There have been rare bugs in the past which made it impossible to start Lightroom until the Preferences file had been reset, so if that is happening you’ll be unable to carry out the above precautionary checks, and will instead have to go straight ahead to reset the Preferences file.
Extract from "Adobe Photoshop Lightroom - The Missing FAQ" by Victoria Bampton
How do I delete the Preferences file?
Deleting Lightroom’s Preferences file can solve all sorts of ‘weirdness,’ so it’s always a good early step in troubleshooting.
On Windows, the easiest way to find it is to go to Edit menu > Preferences > Presets tab and press the Show Lightroom Presets Folder button.
You can also navigate directly to C:\Users\[your username]\AppData\Roaming\Adobe\Lightroom\Preferences\ - AppData may be hidden by default, but you can type %appdata%\adobe\lightroom into the Start menu search box and you’ll be taken directly to the Lightroom user folder.
Whichever way you choose to find that folder, close Lightroom before going any further. In the Lightroom user folder will be the Preferences sub-folder which contains the Lightroom X Preferences.agprefs file(s) (there may be other older versions if you have upgraded Lightroom) - move the complete Preferences sub-folder to another location outside the Lightroom user folder or simply rename it, and then restart Lightroom.
On Mac, you’ll find com.adobe.LightroomX.plist with the other Preferences files rather than Lightroom’s Presets, so you’ll need to navigate directly to Macintosh HD/Users/[your username]/Library/Preferences/. The Library folder is hidden by default, so in Finder hold down the Alt (Option) key while using the Go menu. The user library folder will then be listed below the current user's home directory. As with Windows, close Lightroom and move that file to another folder or delete or rename it, but do not restart Lightroom yet. NOTE: if there are com.adobe.LightroomX.plist files from earlier versions of Lightroom (which will likely be the case if you have upgraded Lightroom from an earlier version), also move or rename or delete those, which will prevent Lightroom from using them as the basis of the new Preferences file that it will create on start-up.
An additional step is now required on Mac systems due to changes made at the OS level (i.e. outside of Adobe’s control). Put simply, the current LR preferences are now stored in the system cache and would then be used in the event of the file on disk being reset as described above. To prevent this, and to force Lightroom to create and use a new preferences file, it is necessary to do a System Restart (to flush the system cache) after resetting the existing preferences file, and before starting Lightroom again.
N.B. If you are comfortable using the Terminal app, the need for the system restart can be avoided by entering the following command (after ensuring that Lightroom has been closed down):
defaults delete com.adobe.LightroomX
Once the above actions have been completed, Lightroom can then be restarted to see if the problem has been cleared.
Moving or renaming that preferences file, rather than deleting it, does mean that you can put it back if it doesn’t solve the problem, to save you recreating your preferences again.
N.B "X" means enter the appropriate single digit version number.
What is deleted when I delete my Preferences file?
When you delete your Preferences file, the obvious settings that you lose are those in the Preferences dialog, but it also includes other details such as your View Options settings, last used catalogs, last used settings, FTP server details, some plug-in settings, etc.
Your original photos, Develop settings, collections, presets and suchlike aren’t affected by deleting the Preferences file.