Did you mean
"...are NOW showing as numbers."
instead of
"...are not showing as numbers."

If yes, then maybe...

When excel saves the file, it saves it as a temporary file with a funny name (8
characters--no extension).  

If the save is successful, xl will delete the original (or rename it to its
backup name (like "backup of book1.xlk)) and if that's successful, xl will
rename the funny named file to the original's name.

Common things that get blamed for interruptions to this process are antivirus
software poking its head in or network errors--either permissions or physical
problems.

Maybe temporarily disabling the antivirus software would be the easier thing to
test.