Hi MickyKK-
You may want to provide more specific info such as the type of ipad you have and what email service you are using. This will help others to respond back to you with an answer.
Have you tried to quit the mail app by pressing the home button twice method? By doing that, it will show the multi-task bar on the bottom of all the apps that were visited. Find the mail app and long press until it jiggles. You will see a red circle-press that to close it out. It will not delete the app-don't worry. Than go to your mail app and see if it works.
If that doesn't work-try to restart your ipad, by long pressing the power switch and unlock the slide button to turn off.
Third-than reset your ipad by holding the power switch and home button at the same time until you see the apple logo.
Hopefully this will help you-Good luck. Let us know if it this worked or not.