Due to their size and the way they operate, rear projection TVs tend to develop more and unique problems than any other type of TV, especially as they age. This does not mean that a rear projection TV is a bad TV or that all hope is lost; it's just that the trade-off for an incredible picture is a little more maintenance than normal.
Take your TV off "Demo" mode. If you are attempting to change your TV's settings but they keep changing back to their defaults, your TV is still on Demo mode. This is a mode electronics retailers use to show customers all the features the TV has. Demo mode can be disabled through the TV's main menu using your remote control.
See if you are having sound issues. If you are trying to use your rear projection TV's digital audio output and have no sound, this is because you have a device plugged into one of the TV's HDMI outputs. Unfortunately, these two types of audio outputs cannot be used at the same time, so you must find another way to hook up the device you were trying to hook up to the digital audio output.
Make sure your TV isn't just overheating. Due to their size and construction, rear projection TVs tend to get hotter in a shorter period of time than normal televisions. If your TV is randomly turning itself off after prolonged use, it is overheating. This is because the internal temperature of the TV is rising to dangerous levels. Make sure the TV's air vent is not covered in any way, and give your TV a rest for a day or two before using it again.
Determine if you have any "dead pixels." A pixel that has died will appear as a small blue or black dot somewhere on your TV screen. If this happens, there isn't anything you can do to correct it at home, but it will be covered under your manufacturer's warranty. Contact your manufacturer about getting it replaced or repaired, at no cost to you.
Check to see if your rear projection TV's convergence chips are bad. These are little chips inside that essentially tell the TV how to operate when it comes to color. If your color settings are drastically different than they should be (for example, everything has a green tint), no matter what color settings you try to use, your convergence chips are going bad. You can replace them yourself using a repair kit obtained at a television repair shop, or you can get it covered under your manufacturer's warranty.