Mail seems to be frozen. What can I do?

My Mail was working fine and then one day when I turned on my Air, Mail was not working and I can't get it back. I can see my old emails but it won't get any new ones, or let me send anything. It seems to be frozen. When I look at connection doctor, it seems to be stalled with copying files but when I press the red stop symbol, nothing happens. There's no way to reboot that I can see. What should I do?