Burning smell or smoke
Shut off the main breaker if you can do so safely, leave the area and call 911 if you see flames. Then call an emergency electrician — do not restore power until the cause is found.
Sparking outlet or switch
Turn off the breaker for that circuit and unplug anything connected. Do not use the outlet again until it is repaired.
Breaker will not reset
Unplug devices on the circuit and try once more. If it trips again immediately there is a short or ground fault — leave it off and call.
No power in the whole house
Check whether neighbors are out. If only you are affected, inspect your main breaker and call your electrician; a lost phase at the meter can damage appliances.
Water contact
If outlets or the panel got wet from a leak or flood, keep power off to that area until an electrician inspects it.