I have had the same problem twice in two days. When trying to send a mail attachment via a 3rd party application (one time it was a photo from Apple's Photos app, the other time it was a PDF from iAnnotate) I have had the app lock up after pressing the send key. When I exit to the home screen and re-launch the app, the app reopens as if it had been force quit and everything appears to be fine. However, when I go into Mail, I find that mail is completely locked up. I just see the mail screen with no text on it (as you see it when you first launch before any of the text is loaded). Force quitting does not solve the problem at that point. Both times I had to reboot my iPad and during the reboot I got stuck at a black screen with an infinite circular loading icon and after about 10 minutes I did a hard reboot from there which allowed my iPad to boot back up again. Once I lost a bunch of settings on the reboot (like the orientation lock switch) which I had to set up again. The second time it happened, I didn't loose any settings. Does anybody have any idea what might be causing this problem?