First thing I would try is double press the home button to reveal the task bar, touch and hold any app icon until the minus symbols appear in the top left of the apps. Now, touch the minus symbol on the mail app. Press the home button to get he home screen back. Open mail and see if it's working?
If it still isn't working, press and hold the on/off button until the slider appears on screen. Wait for it to fully power down, then restart.
Hopefully, mail will be working agin, if not, please let us know and we can try something else.
The Archangel