The crash, and not restarting has happened to me three times this last week. The only way I got ti to restart was to unistall MC and delete the registry entries too. This is an option when you remove MC. However, to keep your library intact, do not check the option that enables deletion of the library. Sounds obvious right?
I found that this allowed me to install MC and finally run it again, without removing my library. In addition, after large import/tagging session, I try to back up the library to a network drive. That way I won't lose as much time in case something happens to my library.