I Can't Save My Current Progress?

Not really an idea, why this happened - maybe someone other.

Check the path if it's the same file that is shown in the start center as the path of the file where you made the last changes. Reload the file with "open" in the menu.

Just a few suggestions concerning your fear loosing the changes again:

- save the score under another file name, (create first a new document, and copy all into this document); close this second music sheet an open it again to check if the changes were saved.
- upload the file into this topic (attachment) or your music profile and reload it

Last but nor least: there is always the possibility to looking for backup files, when you're missing saved changes:

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

If you hit the Save button before closing MuseScore, the file WAS saved...somewhere. MuseScore won't even close without prompting you to save changes to a score first. But the file might not have been saved in the folder/directory in which you're looking for it. On Win7 (what I'm running, too), the 'remember last directory' can mess you up by saving a file to a location that you didn't expect. I don't purport to understand it completely, but unless you're careful to specify exactly where you want something saved, Win7 has a nasty habit of saving to a directory when you thought you were saving to a specific folder within that directory.

Go to the Library and start looking for that file in the directories above the folder level you thought you were working in. Also look in all the directories starting with MuseScore. If all else fails, try to remember a specific and precise phrase from the title of the file itself, and run a system-wide search for it. Set the search parameters to look for hidden files and everything else, too.

Hope this helps....