I have a york heating and cooling unit, it works great when it works. Why would the a/c not work? The fan outside the house runs but nothing is happening inside the house.. it doesn't kick in when I turn the thermostat to cool... I have no idea where to even start looking

Sounds like could be a bad stat, or furnace control board. Is this a ac unit or a heatpump? Does the fan come on in the heat or fan on position?

Yes it is an a/c unit. The outside fan runs and the heat works fine. I did have the same problem with the heat in the fall and had to have a repairman replace a part. Could that have something to do with the a/c not working now? Could the repairman have fixed the heater but messed up the a/c?

What part did he replace? He may have wired something back up wrong.

I really don't know the name of the part but the problem with the heater was the same as the a/c. it would turn on but it was not kicking in. there was nothing coming out of the heating vents and that is the same with the a/c.