I am getting an error message when I try to open the current catalog I am editing. It says "relaunch lightroom with this catalog?" Below that message it says "in order to use this catalog, lightroom must close the current catalog and restart. Do you want to relaunch lightroom (and then it lists the name of my catalog)." I click relaunch and it says "the lightroom catalog cannot be opened because another application already has it opened." I have exited out of the other Lightroom, where another version of the catalog is without edits. HELPPPPP!!!
However you may have thought you exited the program last time, nonetheless it didn't exit correctly and it's still locked for that one use. You must go to the folder where the catalod exists and simply look for a file with the extension: ".lock" and delete it. Then restart and it will work. I see this all the time and certify this is the problem, and the fix.