problems with Mail

I mainly use Gmail but have an older Yahoo account also. I had set up Mail to access my Gmail and to send messages from links directly through Gmail. Recently I have tried to send an email but it doesnt not go through. The status bar is half way and Mail just keeps working but never sends the message. It will not let me stop this process and deleting the message does not stop Mail from trying to send. Even when I shut down the computer, Mail cancels the shutdown.
What have I done to Mail and how can I fix this? It had been working perfectly up to now.

Thank you
Chris