Hello and welcome.
Sounds like a possible corrupted plist. Here's a possible fix:
1. Launch Mail and click Window > Message View in the menu bar to make it appear
2. Click Mail > Preferences... > Viewing and check the box next to Use classic layout, then close the Preferences window
3. Quit Mail
4. Relaunch Mail and the Message Viewer should act as normal
5. Quit Mail again and relaunch it
6. If the Message Viewer window is now working properly, uncheck the box next to Use classic layout and close Mail
It should stick then, as changing the settings refreshes the plist file for Mail.
Let us know if that works for you,
C