Clarification
The mail menu shows inb the upper left corner i go to File do a pull down and select New Viewer Window, Nothing happens cn not get a viewer window. Running Big Sur version 11.1 and Mail 14.0.
In the process of getting the version numbers the mail app did and unexpected quit. Wh ne i go it reopened check a couple other things. under the mail windows pull down found no mail box selected. Went to accounts. my wife's mail account was not showing up. I was able to select her mail provider which start to have me setup her account in the process go a window that let me select her existing account and then was able to get a viewer. Not sure how things got corrupted but looks like I have ist working again.