What do I do if my Ipod touch 4 will turn on, but the touch screen won't work so I can't unlock it?

This has happened 3 times... my Ipod touch 4 will be fine one minute,but then the next time I turn it on the screen won't pick up when I touch the screen it wont pick it up. I can't unlock it or completely turn it off becasue the touch screen won't work, but I know it is still alive becasue when you press the home button my lock screen turns up. if you know how to fix this bug please tell me how.