After High Sierra upgrade, Mail not working

After I upgraded to High Sierra (10.13.1) on my MacBook Pro (Retina, 15-inch, Early 2013), Mail did not open. I went to user/Library/Mail. There is a red minus sign badge on the folder icon. I did a Get Info, and my admin permissions showed a name of "Fetching...". I deleted that permission line, and inserted a new one, with my username and permissions of Read & Write. I then tried to open the Mail app, and got an IMMEDIATE dialog box named Mail Upgrade, with a progess bar and a link to Help. Both buttons (Quit and Continue) on this dialog box are disabled. I have now waited 24 hours. The progress bar is all blue, but the buttons remain disabled. I quit Mail from the Mail menu, waited a minute or so, and attempted to restart Mail. The same dialog box re-appeared. I checked Mail's CPU usage in Activity Monitor, and it is present but negligible (CPU is 94% idle). What do I need to do to get my mail back?

Thx,

Bill