Had this problem too, but there's a way to fix it: If the screen goes blank but the buttons light up, hold down the power/hang up button for about a minute until you see the screen flash, then let go of the button, wait three seconds, and then press-and-release any button (except the camera button). You should get the swipe-to-unlock screen (or the security screen if you have a security-unlock code enabled). This will not reboot the phone.
If you had a Bluetooth headset paired & connected before the screen went blank and froze, you may have to manually disconnect and then re-connect to get the headset to work again. Depends on the headset.