Double tap the Home button to bring up the multi-tasking bar. From there locate the mail app and hold it down until it wiggles. When you see the red minus sign, tap it to close the app.
Do a reset of the phone, holding the sleep/wake and home buttons together until you see the Apple logo and then release. After the phone restarts, see if the mail is fixed. If not, the next steps are a restore from backup and if necessary, restore as a new device.