After the call not able to access the phone

[Android Question]

When i make calls or receive the call my Sony Xperia Arc S mobile display is going blank if i press any key can not able to see the display and can not access the phone Sony Xperia Arc S, that's fine, the proximity sensor turns off the screen but when i finish talking the screen stays black but the phone is turned on and i can not access it, not even pressing the turn off button or the home key. it's like the sensor is always on. i have to take out the battery in order to turn on and use the phone again. So please help me to rectify the problem.PLEASE...