Most of the time this is due to a tripped circuit breaker.
A circuit breaker is a device used to protect the wiring in your electrical system from failure.
Basically they are sized by the size of wire they are connected to.
The size wire dictates what can be connected to that circuit.
Sometimes too many things are connected to a circuit causing an overload and the breaker trips.
The important thing to remember about tripped breakers is that something caused that breaker to trip and that problem should be corrected before further action is taken.
Rarely it is the breaker being faulty that causes tripping it is usually what is connected to it.
It is recommended to get as much information about the tripped breaker( Manufacturer, Amp rating, What is connected to it) and call a qualified electrician.
Most breaker handles have three positions off, tripped, and on.
The handle on this type will move to a center position when tripped and will not be able to be moved to the on position unless moved to the off position first.
There are some breakers that are two position that trip to the off position.
In either situation it should be relatively simple to find the offending breaker.
There may also be an occasion when a breaker may trip and the handle does not appear to move.
That is where it gets a little tricky.
It is important to remember one should always have an up to date panel schedule (directory of what each breaker controls) to ensure one knows exactly which breakers are in use.
No sense checking breakers that are not in use.
After verifying the panel schedule visually check to see if any handles are in the center or off position.
It is important to investigate further what is attached to that circuit and possibly remove some items to reduce load.
A breaker can be reset first by moving the handle to the off position then to the on position.
If the breaker resets, make a note of the breaker in the event the breaker trips again.
If you reduce the load and the breaker trips again, you should then contact a qualified electrician.
Do not reset a tripped breaker a second time without a qualified electrician checking it first.
If the breaker immediately trips STOP! Again make a note of the tripped breaker, what it is connected to and contact a qualified electrician.
Electrical troubleshooting is not to be attempted by the untrained.
Some steps are simple processes and others can get really complex.
If you are unsure or not comfortable with what you are doing then by all means do not attempt to handle electrical equipment.
You can however visually inspect and document your problem and give your electrician as much information as possible.
It is always better to be safe and pay a little money than attempt something you are not trained for.
ALWAYS BE SAFE!