iPad 3 - my mail app is frozen and I can't unfreeze it, does anyone known what to do?

Try closing the Mail app completely and see if it works when you re-open it : from the home screen (i.e. not with the Mail app 'open' on-screen) double-click the home button to bring up the taskbar, then press and hold any of the apps on the taskbar for a couple of seconds or so until they start shaking, then press the '-' in the top left of the Mail app to close it, and touch any part of the screen above the taskbar so as to stop the shaking and close the taskbar.

If that doesn't work then also do a reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.

I have tried rebooting, but it didn't work. I deleted the account in which the outgoing email message is stuck, but that hasn't helped. Right now, an email sent from my default account (iCloud) to myself as a test just seems to disappear--the "sending" bar doesn't appear, the message is not in "drafts," it's not in "sent," and it doesn't arrive in my inbox. Email I send from other accounts to my iCloud account appear in the inbox and in their respective accounts' sent boxes.

followed directions in the second paragraph re sleep/home button reboot and mail is working for the first time in 2 days - thanks!

Sure wish it would work for me.

Fixed, I think! I deleted my iCloud account and re-added it. I've sent an email from that account to myself, and I received it, so I am hoping all is well.