The screen gets switched off during a phone call when the proximity sensor is covered. This is done to avoid accidental touch screen actions when you are in a call. To switch the screen on, hit the power (lock) key.
If the screen gets switched off immediately when dialing a number or receiving a call, check that the proximity sensor on the top of the screen is not covered by any kind of protective film or tape.
You can take the following steps to check the issue. If the screen keeps switched off, the protector may not be of the right size and will block the sensor on the upper part of the front side. Then either remove the protector or cut off the plastic from the blocking area.
- Make or receive a call with the phone.
- Once the phone is in a call place your hand 1cm above the screen and display will switch off.
- Remove your hand and display shall switch back on.
- Repeat the test steps 1,2,3 for 3 times to verify the proximity sensor function.