If MuseScore or your computer should crash, or if power is lost, a pop-up message upon restarting MuseScore will ask if you wish to restore the previous session.
If you click No, any work from your previous session will be lost. If you click Yes, MuseScore will attempt to recover the files that were open.
Behavior of saving after session recovery
When MuseScore recovers files after a crash, it renames them with the full path name added in front of the original file name. This very long name will appear in the tab(s) above the active score window. On some operating systems, when a user saves any of these recovered files, it will be saved in the folder in which the program itself is running. This is not necessarily the same directory in which the scores were saved when they were created. You may not be able to locate the revised file in the usual folder.
To avoid this, do not use "Save" the first time you save a recovered file. Use the "Save As..." menu item before making any revisions to the score, to save each recovered file under either its original name or a new name. This will open a window to allow you to navigate to the correct folder and directory. This is important in order to ensure that the file is saved to the folder in which you expect to find it later.
Finding recovered files
In the event that "Save" is used instead of "Save As..." with a recovered file, you will have to find the files in your computer. The actual location of those files will vary, depending on your operating system, and in which directory MuseScore is installed.
For Windows 7, with a default installation of MuseScore to the x86 program files directory, recovered files are auto-saved to
C:\Program Files (x86)\MuseScore2\bin.
For Windows 10, look in
C:\Users\[User Name]\AppData\Local\VirtualStore\Program Files (x86)\MuseScore 2\bin
You may need to run a system-wide search in order to find files saved directly after a session recovery. Use keywords from the original file name as well as wildcards, and specify the date modified.