The phone is fine. But. If... after I had a phone call on my Samsung i9220 then the screen turns black. If I press the button on the top right on the phones side I can see the "Desktop" for about ~0.5 seconds, but the screen turn black again. What could be the problem? The phone had been reseted several times and I can't find a setting for this in the "Settings". It has Android 4.1.1 and I think there is no update for the Samsung i9220 Android version yet, so it's up-to-date.
The problem could be your proximity sensor, which is located on top of the front of your phone. Basically the way it works is if you're on a call and you put your phone to your ear, the screen will go out to avoid your ear messing with the touch screen. If you are under warranty, have it checked on your nearest service center. I had this problem once, told my service center that I think the proximity sensor is the problem, then they fixed it in under 30 minutes.
Check out this question and install the apps from the top answer and see for yourself: