Usually, when a breaker shuts itself off, the cause is apparent. Whatever appliance you just turned on before the outage is probably the one that overloaded the circuit. Appliances like microwaves, hair dryers, space heaters, and vacuum cleaners can cause a sharp enough amperage spike to trip the breaker. Another common cause which may be harder to identify, is when the air conditioner kicks on while you have other appliances running on the same circuit. The switch on the tripped breaker should not be inline with the others. It should be in the tripped position, which is usually in the middle of the breaker switch position. To reset, simply switch to the off position first then switch to the on position. Make sure to firmly switch the breaker to the off position first. Note just because the breaker is in the on position it doesn’t mean that it is sending the electricity through the circuit. If problem persists call a qualified electrical contractor.