Has your update fixed the problem?
If not, it is likely the proximity sensor. Have you opened the phone up before it started, replaced screen?
Do the following tests.
- take the phone next to a very strong light (for just a couple of seconds, you don't want to melt it) and press the power button. Does the phone wake up?
Medium Complexity (be very careful not to fry the motherboard or screen by static - do at your own risk)
- turn off the phone
- take off the back cover and the top motherboard cover.
- carefully remove the proximity sensor from its nest, so that the sensors are facing the light
- very carefully turn on the phone back on
- once the phone is on, put it to sleep and try to wake it up again either with tapping screen or power button.
- turn off the phone (completely power it down)
If either of these work, your proximity sensor just needs to be wiggled a little in its nest so that it is picking up light. Make sure it has it's rubber 'glove' still on.