Recently a friend of mines iPhone screen would not wake up when he pressed the Home button. He could still receive SMS messages and people could ring him but he just couldn’t see anything on the screen.
After a little testing I discovered that you can perform a hard reset of your iPhone by holding down the Home button and Power Off button at the same time. After about 3 seconds (if your screen is visible) you will see the Power Off Swipe button appear – don’t swipe it, keep pressing the two button for another 2-3 seconds.
Your iPhone will then reset itself and auto-reboot. You should see the welcomed image of the spinning Mac OS X boot icon appear very soon after. Your iPhone should be ready to use once bootup is complete.
If your screen is still not responding after performing this reset, you may well have a hardware problem and need to take it to an Apple store or back to your particular phone vendor.