Files are automatically saved in a directory at: ~/.local/share/Mousepad
The files have no extension, they get the name "autosave-0", "autosave-1", "autosave-2", . . . , "autosave-n".
At some point, after a reboot, if you have not saved any of those files and you try to open the MousePad text editor, it won't come up unless you call it through the command line and you will get the following message:
(mousepad:21465): Mousepad-WARNING **: 19:56:09.717: Some 'autosave-*' files in directory '~/.local/share/Mousepad' do not correspond to any session backup anymore. They will not be deleted automatically: please do it manually to remove this warning.
You can go there and delete those files, or save them, or move them to another directory; they are plain text files, you can open those with VIM, NANO, or any other text editor. After those files are removed from that default directory the MousePad text file app will work fine and it will respond to the calls from the GUI.
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