We end up repeating the usual troubleshooting steps regularly, so here's a quick run down.
1. Make sure you have backups!
2. Restart Lightroom
3. Restart the Computer
4. Delete the Preferences file.
Close Lightroom.
Find the Preferences file.
Windows XP - Documents and Settings/[username]/Application Data/Adobe/Lightroom/Preferences/Lightroom Preferences.agprefs
Windows Vista - Users/[username]/AppData/Roaming/Adobe/Lightroom/Preferences/Lightroom Preferences.agprefs
Mac OS X - [username]/Library/Preferences/com.adobe.Lightroom.plist
These may be hidden folders(see note) on some systems, which you may need to show (or type the folder path into the address bar).
You can move or rename that preferences file, rather than deleting, if you think you may wish to put it back.
5. Optimize the Catalog (Edit menu > Catalog Settings > Relaunch & Optimize and wait for it to tell you it's completed.
6. Create a new catalog to rule out catalog corruption. (Restart Lightroom holding down Ctrl (Windows) or Opt (Mac) and select Create New Catalog.)
7. Import some images into that new catalog to check everything's working as expected.
* Hidden files note: By default, Windows sets some 'system' files to hidden. To address this, Control Panel (classic view) > Folder Options > View tab > 9th item down (usually), enable the radio button on 'Show hidden files and folders'
