I experienced a power failure while running LR 5.7.After restarting my computer, when I try to open LR I get the message that "The LR Catalog named "Lightroom 5 Catalog" cannot be opened because another Application already has it open. Quit other copy before trying to launch". I have tried many things reinstalling LR etc., etc. Sometimes I can start LR by opening a different Catalog but but the basic problem continues.
Go to where your catalog lives - you will see a file there called******.lrcat.lock delete that lrcat.lock file and restart
Great answer. The same thing happened to me just now. Spotted your post, found the lock-file and fixed it.
I keep getting that issue as well. It says to retry or switch to other application and cancel it. When I hit switch to then lightroom simply opens up. Not sure how to take care of that. I was wondering if it had something to do with lightroom mobil on my Ipad.