Mail froze on Macbook

I'm new to the Apple world and don't know how to fix this. Mail's wheel doesn't stop revolving. It was working perfectly well this afternoon. I didn't install anything new, nothing has changed. But Mail seems to have frozen, the rainbow wheel spinning around forever. The only way out is through Force Quit. I've seen a number of solutions on the internet but I'd rather have it from you.
My version is Mac OS X Lion 10.7.5

Thank you tons for taking the time to help me

Have you done a Repair Disk Permissions in Disk Utility ?
Have you rebooted the Mac?

Amazingly, Mac asked to do a few updates, which I did, and Mail went back to normal.
Thanks a lot
Cheers