All of a sudden, my Lightroom is not working even though it was working fine yesterday. Today, it keeps saying "The Lightroom Catalog named 'Lightroom Catalog' cannot be opened because another application already has it opened. Quit the other copy of Lightroom before trying to launch." No matter what I do, it won't work. Can anybody help me?
Its possible there is a .lock (LOCK) file present in the "user" My Photos/ Lightroom sub=folder alongside your Lightroom Catalog File.
When you open a Catalog File in Lightroom the applications create a file with the same name as the Catalog with a .lock ( LOCK) extension. This is to insure there is no other use of the catalog. When you quit Lightroom this .lock file is automatically removed, however if Lightroom closes unexpectedly e.g power loss/ crash then the .lock file might remain preventing any further use of the catalog file.
Check in your user / my Pictures (Pictures) / Lightroom sub folder and see if there is a .lock file alongside the catalog file. Delete this file, only the .lock file the try and re-start Lightroom.