Using El Capitan, it works like this. Sometimes when using a template, the window freezes and can not be closed. If you quit and re open Mail, the number of messages appears encircled in the icon, but the main window is completely blank.
This relates to inner peace to the extent that it is disturbing to lose your e-mail and have everything blank, no windows, no messages, nothing.
To fix this, go to System Preferences, select Internet Accounts and open that preference pane. Click on each account in turn and “un check” or de select “Mail.” Then close System Preferences and open Apple Mail.
Your window should now appear, where before all was blank, but your e-mail accounts do not appear yet. We’re almost done. Close Mail. Go back to Internet Accounts in System Preferences and enable each account in turn by clicking the box next to “Mail.”
That should bring everything back to normal.
© 2019, daniel_dreifus. All rights reserved.