I had the same issue a while ago and it was solved by this:
Look for pidfile.tmp (or something similarly named) in the Mopy directory. Delete it, and Bash should start.
Bash uses that file to prevent you from opening multiple instances of Bash. It's created when Bash is started, and deleted when Bash exits. If Bash crashes, or you forcibly close Bash, the file remains behind. Depending on your luck*, Bash may not start again until you delete it.
*it actually depends on what process id your operating system assigns to the new Bash instance...