Screen going black when I press dial button in Lenovo P770 Android 4.1.1

After press call screen goes inactive When I press the dial button in the mobile screen the screen goes inactive but the call is in-progress at back end, It does not come back on when I move it from my face, only when the call has ended by the called person. I cannot check call waiting, internet, contacts, call duration, use speaker phone or anything during a call. Before the problem when I press the dial button the screen is active condition when I place the head set near to my ear the screen is in-active i.e. screen light is off then I bring the head set out of my ear suddenly screen is active i.e. screen light is glow THIS OPTION IS NOW NOT WORKING, How I can overcome this problem. **NOTE In Simple** My screen goes black when i press call button & Screen works again only when the call is finished. I know about the feature when you put the phone on ear it goes black and the second you remove the phone from your ear , the screen turns on again. But now is different My phone model is Lenovo P770 OS Version - Android 4.1.1