Mail is frozen. Can't open or close the application. Help!

I have a fairly new (as of December) macbook pro. Had been using it without any issues until a couple of weeks ago when I was suddenly not able to open Mail to get my email messages. It's like the application is frozen. I tried to shut down the computer and can't because a message keeps popping up saying that I have an application still open (Mail). Same goes for trying to install updates. I've tried to close it, force quit the application, turn off the comupter without shutting it down....and nothing works.

So, I can't open Mail, but I can't close it either.

Anyone know how to retrieve Mail from the apparent limbo in which it's residing?

Thanks!