I can hear the unlock and lock sounds but no display on my iphone 4?

I can't see the display on my screen but the iphone works

This is an issue with your screen. The phone is fine but the screen has either disconnected or has "died". I would suggest bringing in your iPhone or sending it to http://www.appletreecanterbury.com

I have found them to be the cheapest in the UK and the best service.