This service call will usually come in as "We are in a power outage and the generator doesn't start." First, verify that nobody has accidentally pushed a remote emergency power off switch.
If a breaker trips after the automatic transfer switch, the generator will not start. The status of the automatic transfer switch should be checked during a power outage. The ATS should have some kind of lights or display showing the switch position and source availability. If a breaker is found to be tripped, make sure you can determine the cause of the trip prior to resetting.
Never try to manually operate an automatic transfer switch if you do not know how to do so properly. Severe bodily injury or immediate death can occur. The transfer switch is smarter than you may think and has a specific reason for being in the position that it is in; attempting manual operation may drag you into a live bus if it is done incorrectly.