Do you mean air conditioner by AC, or all the alternating current electrical system? Is this a built-in generator or an external portable generator?
If it's all the power, as enblethen said, check the circuit breaker on the generator--true for both built-in and portable generators. Don't rely on a visual check, but flip the breaker off and on (if it's a lever kind) or otherwise try to reset it. Some look to be engaged even after they trip.
If it's all the power and it's a portable generator, possibly you have an EMS installed and it sees the unbonded generator as an open ground and won't connect the power.
If it's just the air conditioner, does it try to start? If so possibly your generator just isn't supplying enough power to get it to start.