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