This happen to be b/c something got screwed up with the Mail app always got hung up or something.
Things to try in order of easy to hard:
1) Just reboot the phone. Turn it off and back.
2) Try a forced reboot by holding home and lock button.
3) Do a restore using a backup
4) Delete your mail settings and enter them again
5) Do a restore as new iPhone (you will lose all settings, but this should fix it for sure if the above don't)
Good Luck!