If you use your car audio system intensely, you may find your amp cuts out. Over heating in some models causes permanent damage, yet, some models (usually from Pioneer) have a function that turns off the amplifier when overheated, protecting it from damage. If you have a model with this function, simply wait till it cools down and resets itself. If you don't have this protective function, simply let it cool down and test it out again. If it works, try to use your car audio system a bit more care...