Lost Score

That file is just 180 Bytes of zeros, and as such broken beyond repair.
See https://musescore.org/en/node/52116 and check for backups, created by MuseScore.

And of course check backups you created by yourself, or have you really workED on a file for half a year without having stored in in different place? What if your harddisk had dies during that time?
Some Cloud Service like Dropbox, box.com, Google drive, Microsoft OneDrive, itunes, etc. or even 'Save online' on MuseScore.com, all for free, or USB Thumbdrives and SD cards should do.

I'm using MuseScore since almost 8 years, have written more than 500 scores, store them on box.com (used to use wuala and dropbox before that), on in my personal homepage via ftp, on several USB drives and SD Cards and as of newest on MuseScore.com. I've never ever lost a single score, so never had to use any of these backup, except for some unintended edits I noticed too late to revert directly.

You are going to have to resort to looking for a backup version of the score.

This will give you a little bit of info to look for https://musescore.org/en/handbook/file-formats-0

This will probably help more https://musescore.org/en/handbook/recovered-files

I hope something is still on your computer but I have little hope. The only other thing I would suggest is that you see if the is some undelete utility available for your operating system since most OS's save a new file by renaming the original to something else, then saving the new version then if necessary hide and then mark as deleted the original.

That attachment is not mine. I have no idea where it came from or how it became attached to my post. I have never seen it before. Nevertheless, thanks for all ideas!