I am using LR 6 on my PC, Windows 8.0. Just recently encountered a problem where I cannot access onof my LR catalogs. The error message from LR says that the catalog is open in another application. Close the "other version" of LR, then relaunch to open the catalog. I cannot find any other version of LR running on my PC. ????
In the same folder as your catalog file is a file whose name ends with .lock
Delete this .lock file
DO NOT DELETE ANYTHING ELSE