If the circuit breaker to your indoor or outdoor unit trips, you may reset it. Turn it to the "off" position if it isn't there already, then back to the "on" position. If the breaker trips a second time, then you should schedule a service call.
The breaker is tripping for a reason. It is protecting the equipment, the wiring, and the house. Do not just keep resetting it and ignoring the situation. Yes, it is possible that maybe it is just a bad breaker or maybe it was just a thunderstorm. But you are dealing with high voltage and amperage and possibly high temperatures which could cause melting of wires and possibly a fire.
One very common cause for breakers to trip is loose electrical connections. Larger aluminum wires tend to expand and contract with the weather causing the connections to loosen over time. It is just a matter of tightening the connections.
Another common cause is a direct electrical short in the wiring or equipment. This is identified by the breaker tripping instantly, once the unit attempts to start. In this situation, do not even try to reset the breaker again. Call for service immediately.
Sometimes the breaker for the outdoor unit trips because the compressor is having trouble starting. It could be old or weak or just tight and a "hard start kit" will do the job getting it going again. Or it may be as simple as replacing a bad capacitor which helps start the compressor.
Below is a list of possible causes and things to check. Items in blue usually require a service call. Items in red however can be addressed, even fixed by the homeowner.
The bottom causes in red are common problems and can be addressed by the homeowner. Try to check for these conditions first before calling for service.
Note - a circuit breaker should not feel hot to the touch. You can touch the breaker with the backs of your fingers. The breaker should feel room temperature or warm. If it is hot, this could indicate a problem. Do not ignore it.