This happened to me yesterday too. I was writing something in the search field when the program suddenly became unresponsive, and when it regained consciousness, the genre of all my songs had been changed to "Blues". Beethoven: "Blues", Erasure: "Blues" etc. Very disturbing. I did not have the "edit window" open, so unless there's some sort of keyboard shortcut for changing tags like genre, I don't understand how this could happen.
I was unable to fix all the unwanted changes using the undo function, but luckily I found this forum thread before shutting the program down, and the MusicBeeLibrary.bak file hadn't been changed since I opened the program earlier in the day, so I was able to restore everything that way. However, despite all the correct genre tags now being back within MB, the "Blues" tag remains embedded in all the actual music files in windows explorer, and I was wondering if there's a way to make MB rewrite all those tags in one fell swoop. If all else fails, I think I can just change all the tags into something else and then change them back again (for instance change alle my "rock" songs to "jazz" and then back to "rock" again), but I suspect there might be a smarter way of going about it. Any suggestions?