A warning message says I have another copy of Moviestorm running!

It is not a good idea to run more than one instance of Moviestorm at once - it can corrupt the saved data for the movie you are currently working on, and will slow down your system, as Moviestorm is quite a large program.

When you start Moviestorm, it saves a file to indicate that Moviestorm is already running. When you close Moviestorm, it deletes that file.
If you start Moviestorm and the file is there already, it thinks that Moviestorm is already running, and warns you about it.

If the file is there because Moviestorm previously crashed, then you can safely start Moviestorm anyway.