Originally Posted by 10fords

Remove the 2 wires(assuming it is a 240 volt unit) from the breaker and see if it resets. If it does than the problem is in the wiring or your AC unit. If it doesn't than it is the breaker. The fact that you saw a spark and the breaker won't reset almost certainly indicates it is not the breaker being defective. Unfortunately, in 25 years of electrical troubleshooting, it is rarely just a bad breaker as that would make it way too easy to fix! If you can post a picture I can tell you what brand of breakers you have and if they are one of the few that have a history of failure- Zinsco, Bulldog, FPE, etc. If you are not well versed in electrical just shut off the main breaker before you work on anything to be on the safe side. Is this a central AC system or a plug in window unit?