Electricity is Off but Circuit Breaker Isn't Tripped
When the lights go out in one or more rooms or in a series of electrical outlets, the culprit is sometimes a circuit breaker or a fuse has blown. However, in some cases the breaker or fuse may seem fine, and even if you reset the circuit breaker the circuit is still dead. In those cases the culprit might be a GFCI outlet that has tripped.
This phenomena can occur in homes that use GFCI (Ground Fault Circuit Interrupter) outlets. The GFCI outlet (sometimes also called a GFI) is like a circuit breaker and can shut off the flow of electricity to the outlet and all outlets, lights and hard-wired electrical devices downstream of the outlet. When the GFCI senses an electrical imbalance it will trip, much more quickly than a circuit breaker. Its purpose is to protect people from electrocution, something the average circuit breaker can't do.
One thing about GFCI outlets, is that they can control outlets and lights in other parts of the home other than where the GFCI is located. It is quite common for a GFCI to be tripped in a bathroom, kitchen, laundry room, closet or even outdoors and somewhere else in the home is also affected. Don't assume the GFCI is even on the same floor and the affected area.
How To Reset a GFCI Outlet
Before electricity can be restored, the GFCI must be reset. However, even before you do that, you must take steps to ensure that it is safe to do so. Turn off or unplug all of the devices that are plugged into the circuit. Make certain no dangerous condition exists before restoring power.
Press the "Test" button. If it does not click, then the ground fault interrupter has tripped. Pressing the "Reset" button until you feel it click should restore power to the outlet and all lights and outlets downstream from this outlet. If it does not reset, the problem that caused it to trip may still exist. Turn off all light switches and unplug all electrical devices on the affected circuit, then try resetting the GFCI again. If the GFCI does not reset, try turning off the circuit breaker for the affected circuit, then reset the GFCI, then turn the circuit breaker back on. If the GFCI fails to reset or immediately trips again, the outlet may need to be replaced or a more serious electrical problem may exist and may require the assistance of a licensed electrician.
Follow this link for more information about GFCI Outlets