A circuit breaker which has been tripped will either be in the middle or "OFF" position. Locate the tripped circuit breaker and reset it by pushing it all the way to the "OFF" position and then back to the "ON" position. Often when you can't cannot reset the circuit breaker, it is because it must be turned all the way to the "Off" position first.
Electricity should now be restored to the circuit. If the circuit breaker trips again before you have turned anything on or plugged anything in, the breaker itself may need to be replaced or a serious wiring fault may exist. Immediately consult a qualified electrician.
If the circuit breaker trips after plugging in or turning on a device, that device may have a short or may be placing too much of a load on the circuit.
If no circuit breakers were tripped and you still do not have power at an outlet, there may be a wiring fault, the outlet may be defective or it may be on a GFCI controlled branch circuit. Refer to the guide for checking a GFCI outlet.