Hello! First off, I'd play with the brightness setting. I'm sure you've already done so and that's why you're here!
Chances are good that you've had the TV for a little while. DLP televisions are basically projectors like a VGA projector or those overhead projectors we used to have in school. Over time, the bulbs will go dim and eventually die. Most DLP televisions need a bulb replacement every few years, depending on how often you use the set and the brightness level. Older DLP televisions generally had a 1,000 - 2,000 hour life, while newer DLP televisions can see anywhere from 6,000 to 7,000 hours of life.
I would suggest replacing the bulb first and foremost. Prepare for sticker shock on these, as the bulbs can be expensive ($100-$300). If you've had the TV for less than a year, then there may be an issue with the circuitry inside the TV causing the issue. Do you have a project repair or replacement plan for the television of some sort?