Why is there no display on my screen?

My screen went blank after a call. I tried restarting the phone several times but there is still no display. The light on the screen as well as the sounds work perfectly. In fact those who try calling my phone can still reach me and i can hear the phone ring but there is no display whatsoever.However, when I'm in a dark place and I double tap the screen or touch the power button the screen lights comes on but there is still no display. What could be wrong?