Notes are resolved by creating a new record with a resolved status, so
sounds like a notes file must have been deleted (this can happen if the
file becomes corrupted).
Fixing this would require restoring the file - this possible by getting the
correct version of the file from project history using a tool outside of
Paratext.
If you send me the project name, I can look in project history for further
clues.
John+Wickberg
Paratext Support