Cant open score

Have you saved them on musescore.com?
If so: click on your name at the top right of this browser window and select "My Scores". If you find them there, you can copy a link and try to open them. If the same message appears, look in your downloads folder, maybe it's there. Then attach it here.
Without having the mscz file, it's hard to say anything about your problem.

Otherwise look in these folders (Windows) if there is a backup copy:

C:\Users\{YourUserAccount}\AppData\Local\MuseScore\MuseScore4
C:\Users\{YourUserAccount}\AppData\Local\MuseScore\MuseScore4\cloud_scores
C:\Users\{YourUserAccount}\AppData\Local\MuseScore\MuseScore4\cloud_scores\.mscbackup
C:\Users\{YourUserAccount}\Documents\MuseScore4\Scores
C:\Users\{YourUserAccount}\Documents\MuseScore4\Scores\.mscbackup
C:\Users\{YourUserAccount}\Documents\MuseScore4\Cloud_Scores

Remark: The AppData and .mscback folders are hidden folders.