There's a 10 year old thread here that has lots of ideas, especially:
Here is the solution that I came up with... First close all of your esri windows to make sure the file in question is not in use. Then delete the lock file in Windows Explorer. Change the name of the original file (put an "a" after it or something similar), this is because if you have recently worked with the file then ArcCatalog will try to go directly to the corrupt geodatabase, but if it has a changed name then the error won't come up. Anyway I am getting ahead of myself. With all the ESRI windows still closed. open the copied version in MS Access. It will tell you that the file you are trying to open is either not a database or it has become corrupt and it will ask if you want to fix it. Say yes. Then open that file up in ArcCatalog and things should work. I hope this helps.