Was working fine. got the following message: The LR catalog named "LR 5 catalog" cannot be opened because another application already has it opened.
Quit the other copy of LR before trying to launch. ???
The catalog may not have close correctly last time you used it.
Go to your LR folder (the default location is normally Pictures)
If you see a file with the word LOCK delete it.
Find the folder that contains your catalog file.
In that folder is a file whose name ends with .lock. Delete this file whose name ends with .lock. Do not delete anything else.
Thank you!! you saved my day!