I have been getting emails but recently I switch on and just find blank screen but the app says I am getting mail this is now happened to me twice any ideas?

Try a reset:

Hold the Sleep and Home button down for about 10 second until you see the Apple logo.

Quit mail like this.

Go to the Home screen and double click the Home button. That will reveal the row of recently used apps at the bottom of the screen. Tap and hold on the app in question until it wiggles and displays a minus sign. Tap the minus sign to actually quit the app. Then tap anywhere on the screen above that bottom row to return the screen to normal. Then restart the app and see if it works normally.