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The Lightroom catalog names Lightroom 5 cannot be opened because another application already has it opened.
I have never seen this message before. It says to quit the other application but I have no idea what it's talking about. Access to ALL of my previous Lightroom work is denied.
When you launch Lightroom, a lock file is created in the same folder as the catalog. The lock file prevents other users from accessing the catalog.
When you close Lightroom, the lock file will be deleted, but if LR crashes or freezes, or there is a power failure, the lock file may not be deleted.
The presence of a lock file when trying to launch LR will trigger this (confusing) error message.
The solution is to delete the lock file using the File Explorer. Do not delete anything else.
In your case the lock file will probably have the name Lightroom 5 catalog.lrcat.lock