Hi
Normally when you are in a call and you lift the phone to your ear, the screen is turned off automatically. This is done to prevent your east from pressing on screen buttons during the call. It should come on again when removed from your ear. This is done by the proximity sensor detecting the closeness of your head. It will only check this and turn off the screen during a call.
So, is there anything which could be obscuring the proximity sensor, a screen protector, a phone case, or even some dirt?
You didn't day what phone you have, but the sensor is normally located at the top of the phone near the loud speaker