It has occasionally locked up and all I had to do was just close it and re-open without any issues. Today the entire computer while running LR locked up and I had to turn off the computer and reboot it. Only thing is that everything else works but LR. When I try to open it it says it is running in another application and I need to close that before I can open it. HUH? I'm thinking something is hung up in the program from the lock up and it may be corrupted. Who knows but I cannot get the program to open and have no idea how to close that "other application" This puts a big halt to my editing.
In the same folder as your catalog file is a file whose name ends with .LOCK (might be lower case letters)
Delete this .LOCK file. Do not delete anything else.