Close the mail app down:
- Press the Home button two times quickly. You'll see small previews of your recently used apps.
- Swipe left to find the app you want to close.
- Swipe up on the app's preview to close it.
Reset the device (you will lose NO data or settings)
Device Reset (won't affect settings/data/music/apps/etc)
1. Press and hold (& continue to hold) BOTH the Sleep/Wake button & the Home button.
2. Continue to hold BOTH (ignoring any other messages that may show) until you see the Apple logo on the screen.
3. Release BOTH buttons when you see the Apple logo and allow the device to boot normally.
Open the Mail app and try again.