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Start out by identifying the on and off direction stamped onto the circuit breaker body. Push the handle toward the off indicator and overcome the spring action and feel for a click. Turn the circuit breaker on. If you are attempting this at your main breaker make sure to shut off "all" the downstream circuit breakers before you begin.
If its a single pole circuit breaker and is controlling lights or a few receptacles, unplug all devices in the affected area and turn the light switches to the off position. At your own risk try resetting the breaker one more time. If it resets this may help identify the problem. If it trips again call for help. If its a two pole breaker controlling larger loads which has tripped, get help...trying to reset a larger size breaker over and over will compound the problem and cause further damage. Electricity is dangerous and should be worked on by a licensed electrician
Most likely it is a failing breaker that is being called on to supply power. It could also be an indication of something loose that is vibrating. A breaker that is warm may be an indication that your panel is overloaded. Any unusual sound or smell from your panel should never be ignored. Have us check it out for you.
The circuit breakers job is to protect the wire in case it was approaching an unsafe amperage or temperature. If your 20 amp microwave circuit was tripping and you installed a 30 amp breaker what would happen is the breaker would allow the wire to heat up way past its current carrying capacity and it would burn up. The correct procedure would be first to find the cause of the overload, then plan the proper corrective action. Never install a larger circuit breaker without understanding the implications of your actions.
Flipping your circuit breakers on and off occasionally is recommended as a form of maintenance. It will help keep the internal mechanical parts operational and may alert you of a defect developing in one of the circuit breakers. Sometimes a tripped circuit breaker gets hung up or stuck in what appears to be the on position even though it has tripped. If you are experiencing a no power situation and suspect a tripped circuit breaker always give them a little nudge to see if they are locked in.