Why does my screen keep going dark?

My screen has started to go completely dark on my G2. At first it would fade and I could still see and use the icons, but then it just goes completely black and I can't do anything with it. It will still ring and I can get messages, but I just amnot able to answer my phone or see my messages. I have to push and hold the power button on the back until it goes off. When I power it back on, it works for awhile before going out again. Please assist. Thank you