Apple mail preferences not working

-
I'm using Mojave on MacBook Air 2018. Mail preferences were working OK, but now they are not. I deleted "com.apple.mail.plist" (as recommended on this site), but that had no effect. When I select Mail Preferences I don't get the drop-down menu, I just get a small rectangular box with the word Signatures on it, and no signatures or any text other than the word Signatures.
I'd be grateful for any help solving this problem.
Jeremy
See more