Q: Sierra Mail hangs up

I loaded the Sierra operating system and the problem I have is that on starting mail I get the mail upgrade screen and it just hangs and does not open a mail window.  However I have another user set up on my machine and that account had no problem with the mail upgrade and opening mail and loading old and new emails.  This second account had a lot less old mail.  I suspect my mail data in my mail account has become corrupted.  It is not clear to me how I can reload mail data from time machine other than returning the entire system back to before the new OS was loaded.  An suggestions?