Sounds like a bad screen inside, these projectors run of a tiny little pc screen with a massive light behind them and a lens in-front, sounds like the little screen is buggered and im afraid to get a replacement would cost more than a new projector.

Although it might be worth making sure that something hasn't come loose and is just sitting between the lens and the light, have a look in the lens WITH THE UNIT OFF so u don't blind yourself and see if it looks like something is blocking half of it.

Also the half that is not showing is it white or black?