I get the message that the Lightroom Catalog is open in another application. I've been searching for answers to this issue. Usually, it says you have to delete the locked file. But when I do that and then try to start Lightroom, I get the following message: "Lightroom encountered an error when reading from its preview cache and needs to quit." It says it will try to fix the problem but it doesn't. I then restored the locked file that I had deleted. I tried to open the file in Lightroom (right clicking and then hitting "open with") and now it's just a .lcrat file. It doesn't say that it's locked. I'm not sure if I made the situation worse by doing that. I have no idea what to do at this point. I appreciate your help.
Delete the .lock file.
Delete the preview cache (its a folder whose name ends with .LRDATA in the same folder as your catalog file — do not delete the .LRCAT catalog file)
That worked. Thanks!