I have a TV, DVD player, and satellite receiver box on one breaker and it suddenly tripped. I went to the panel and tried to reset the breaker and it trips immediately. Then I unplugged everything and it still trips instantly. It's a 20 amp circuit, so I don't think it's overloaded. Does anyone know why this could happen? Should I be concerned?
You either have a short in the circuit, a short in a component you didn't unplug on the circuit , or a defective breaker. The last is highly unlikely, though not impossible. You really need a licensed electrician.
Does anyone know why this could happen?
You are either experiencing a short circuit, or a faulty breaker. Note that if the breaker is an arc-fault or GFCI breaker you may also be experiencing arcing or ground faults on the circuit, which are slightly different than shorts and may not be as easily diagnosed using the following method.
Should I be concerned?
Yes, but you shouldn't be worried, you are being protected. One situation I discovered similar to yours was someone used a large head screw to fasten an electrical box to a stud. The screw head was on the side of the electrical box, and they hadn't tightened down the screws on the outlet, so they stuck out more than they needed to. Apparently this was fine for years, but ultimately they came into contact with each other, the circuit hot was grounded, and the breaker tripped.
So as odd as it may seem, it can happen where poorly done work leads to a failure much later.
In order to isolate the problem I followed a fairly straightforward process. Here I have to caution you - opening your circuit breaker box, electrical boxes, and diagnosing an electrical problem has significant risks.
Loss of life or limb is possible - proceed only if you are knowledgeable and experienced in electrical work. Merely asking this question suggests that you are not, though, so I'd suggest that most people with this problem should consult with an electrician.
That being said, if you have a multimeter you can do what I did:
First, find out if the circuit is still shorted out. Using the appropriately insulated tools, remove the wire from the breaker, and then use the multimeter to first test the voltage, then test the resistance on that circuit. Testing the voltage first is important to make sure no previous electridiot wired two separate circuits together, as well as make sure it's not shorted to some other active circuit, for instance through a mis-wired 240V plug and connected appliance. If there is voltage, stop and get an electrician - diagnosing a live circuit isn't worth it for a DIY'er. If you've unplugged and turned off everything, then it should be high resistance. If it is, then chances are good the breaker is faulty. Replace it and see if that resolves the problem. If the circuit is very low resistance, then you either still have something plugged in or on, or the circuit itself is shorted. Cap the wire at the breaker panel to ensure it doesn't touch anything else, becoming energized while you are working on the circuit.
Follow the circuit from there to each following electrical box. By removing the outlet/fixture/switch/etc from the circuit and testing the resistance on the outgoing wire you can locate the short in the circuit. It'll more likely be at an electrical box, but occasionally it'll be in a wire run itself. I've seen instances of wires being worn through with nearby sharp metal edges (again, shoddy electrical work, or later bad remodeling), and in one case a ground fault breaker tripping because a cable was getting wet and apparently had a tiny cut in the sheathing (nothing nearby could have caused it - was likely due to mishandling during installation).
Replace the bad component or resolve the short, once found.
Could be rodent damage causing a short circuit somewhere in the middle of the run.