Mine had the same problem when i left my sd card adapter in my phone without my card in it. put my card back in it or took the adapter out while the card was out and it works perfect.

Start -> Settings -> G (More) -> C (Power Management). Arrow to "Backlight timeout on battery", left/right arrow to 5 or 10 seconds. May I also recommend finding and installing Candlelight. It's a nice little application to run your light at a dimmer level to save the battery.