by Ken Task.
You won't find .mbz files in moodledata by their humanly recognized names.
Check moodledata/temp/backup/ for contenthashed named directories ... bunch of letters/numbers. See what's in them. If you see a backup.mbz file in one of them copy that out to another location. It's a valid backup. To know if that is for the course you were working with there should be a moodle_backup.xml flie ... in there the course ID and other course settings like it's name.
Fingers crossed!
Might check web server error logs to see if there is a clue.
'SoS', Ken