I lost all my work on a score

see https://musescore.org/en/node/52116

Most likely you saved the score to different folder than you think you did and your score is there waiting for you in that other folder. In order to help more, we would need more information - what OS, what version of MuseScore, whether you did all your work in a single session or whether you closed the score and reopened it at some point, whether there was a crash at any time and whether you were asked if you wanted to recover the previous session and whether you agreed, etc.

This is probably too late to help you, but in case someone else sees it, also check out this thread:

https://musescore.org/en/node/56971

It has to do with autosave and windows permissions and I have no flipping clue how I lost my work (I was saving, all night, to two different file locations, and when I reopened after a crash, both were the same 5+ hours old version). But the file in the location specified in that thread (C:\Users\\AppData\Local\VirtualStore\Progtam Files (x86)\MuseScore 2\bin\) was the newest version. It's now saved to a flash drive, too :)

jw