Welcome to iPhone Forums, Tonycovers!
Do you use the built in Mail App?
If not, remove the app from your device and reinstall it afterwards. Maybe that will help.
If it's the Mail app:
Force close the app - double tap the Home button to show the App Switcher. Then slide the thumbnail below the app icon towards the top of the screen.
Reboot/force start your iPhone. Press and hold Home and Power button simultaneously until you see the Apple logo. As soon as it's visible, let go of both buttons. Your iPhone will power up on it's own.
Another option would be to remove the email address which causes the problem, and then set it up again on your iPhone.
Hope that helps.