Try starting Mail while holding down the Shift key.
That should prevent it from trying to open a mailbox and message.
If it opens, select a mailbox. If it crashes or hangs.
Open again holding down shift key.
Drag the divider between the message list and preview pane all the way to the edge so the preview pane is closed.
Select the same mailbox.
The first message in the mailbox might be corrupted and cause mail to hang or crash.
You can try to open it by double clicking, or try to view it from your email provider's web portal. You may have to move or delete the message to get Mail to work correctly.