I have a Monster Power outlet for my home theater and it began to buzz, which the owner's manual said could be caused by not being plugged in a grounded outlet. When I check the outlet, the top plug does not work, but the bottom plug does. The house is just 5 years old. How can I tell if the outlet has become ungrounded? How can that happen suddenly? How can I fix the outlet, as it is the only plug available to use for the home theater?

You can test the outlet using a relatively inexpensive tester that you can get at any hardware or electrical supply store (Lowe's, Home Depot, etc). You plug it into the outlet and LEDs on the tester will tell you whether its grounded, or has other wiring issues.

As for the top outlet not working at all - are there any wall switches that may control it? Again, the tester will help sort it out for you. I had a similar issue once with an outlet in a relatively new house that I moved into in TX, and it turned out that the builder had wired two hot leads to the top half of the outlet - one switched and the other not (no neutral at all). If the tester reveals a major flaw like that, you should hire an electrician to sort it out for you.

You could easily have a split receptacle.

The bottom of the receptacle is a constant hot. The top is usually controlled by an adjacent wall switch.

Might I suggest that you purchase a multi meter rather than one of the circuit testers. This will allow you to actually see the voltage output that the receptacle is delivering.