Unfortunately, no. I've suffered the same problem you just encountered when arranging my PMD Sky Tower piano solo and LoZ OoT "Ganondorf Battle", and it was very frustrating having to re-write all of it. I know Finale does save ".BAK" files, but these aren't recovery files, they're just back-ups whenever you save an .mus in case of file corruption.
...Sibelius, on the other hand, does have file recovery features
