Try these steps as needed:
1. Reset phone (no data loss): Press both home and power buttons for at least 10 seconds, releasing when the Apple logo appears.
2. Delete and reinstall any Exchange email accounts.
3. Restore your phone in iTunes (or, Settings > General > Reset > Erase All Data and Settings) and DON'T restore any backup from iTunes or iCloud. Your problem is corrupted data or settings in the backup.