GFIs or GFCIs    (Ground-Fault Circuit Interrupter: explanation below after the Helpful Hints info)

Helpful Hints: Do you have some lights or outlets not working?  A GFCI outlet or GFCI circuit breaker may have tripped. 
(Common areas affected by GFCI outlets include: bathroom outlets, garage wall outlets, outside outlets,  outlets in unfinished basements.   Kitchen counter outlets also now have GFI outlets but they operate independently from the bathroom, garage, etc.)
Also read the sections: "Overloaded circuits" and "Short circuits".

• If the lights or outlets in or near a bathroom are not working, look in all the bathrooms for a GFI outlet.  GFI outlets have a [Test] and a [Reset] button on them.  Often lights and outlets are on separate circuits.  This means even if the lights in the bathroom work, the outlet may not.  And you may even have more than one GFCI outlet.  If you have trouble finding them, try looking in the basement, garage or even outside! (Some contractors tried to save money by putting all the required GFI-protected outlets on one circuit and put the GFI outlet itself closer to the panel.)

• Press the [Test] button (sometimes it's black) and then the [Reset] button (sometimes it's red).  If when you press the [Test] button you hear a loud click, then the outlet was already in working condition and it had not tripped.  Be sure to reset it with the [Reset] button.
  You need to press the Reset button in quite a bit to reset. Your finger may not be enough. Use something like a pen or similar to push it in completely

• A GFCI circuit breaker looks different than the normal circuit breakers.  Unlike most breakers, it has a [Test] button on it. To reset a GFCI breaker, be sure to click it completely OFF, before pushing it back to the on position.  If it will not stay ON and shuts itself off, there may be a problem with the breaker, the circuit or something plugged into the circuit.

Safety Information

• It is possible to connect a GFI outlet backwards and therefore, not be protected. It should only be installed by a qualified electrician.

• Any outlet can be replaced with a GFI outlet.

• A GFI outlet can protect not only itself, but also other outlets connected to it.

• GFCI circuit breakers can be installed to protect an entire circuit.

  
  GFI outlets (or circuit breakers) have a [Test] button on them. You can check if the device is functioning properly by pushing the [Test] button, usually black, and a click will be heard. Verify that the outlet no longer works, then reset the outlet with the [Reset] button (usually red).
  You need to press the Reset button in quite a bit to reset. Your finger may not be enough. Use something like a pen or similar to push it in completely

General Background

Ground Fault Circuit Interrupters (GFCI or GFI) are special electronic devices to protect people from fatal electric shocks. Note, however, that you can still get a shock. There's an important difference between these devices and circuit breakers or fuses. Breakers and fuses are designed to disconnect power from a circuit when there is too much electrical current flowing. If too much current flows, the wires will overheat and create a fire hazard. Most circuit breakers are 15 or 20 amps, this equals 15,000 or 20,000 milli-amps. The bad news: it can take as little as 10 milli-amps to fatally shock someone. The good news: GFIs are designed to shut-off when it determines that only 5 milli-amps are "missing", presumably it could be shocking someone.

GFIs were first instituted into the electrical code requirements for bathroom outlets in the early 1970s. Over time, they have proven their effectiveness and have become required in more an more situations. Now, GFI protection is required in areas where the potential for shocks is quite likely: bathrooms, kitchens, unfinished basements, garages, outdoors, jacuzzis, hot tubs.

According to when your house was built, or when work was done in certain areas, you can expect to find Ground Fault protection in the following areas:

Starting in
  Year         Location
1973                 Bathroom outlets
1973                 Outside outlets
1987                 Garage outlets
1987                 Outlets in unfinished basements, crawl spaces
1993                 Kitchen counter outlets within 6’ of sink
1996                 All kitchen counter outlets

Remember: Outlets in one location may be protected by outlets in another location. If you have an outlet not working in one of these areas, check the other areas to look for the resetable outlet with push buttons.  And don't forget to check the circuit breaker box to see if there is an unusual GFI breaker with a push-button there.