The first thing you want to check is in the panel box you should have a couple of circuit breakers clearly marked for you AC and the furnace. Regardless of which position they are in, you should turn them OFF for a few minutes and turn them ON. If the circuit breakers trip again when you turn on the AC then leave them OFF and call for service. If the circuit breakers do not trip just give it few minutes and you know you fixed your problem.

Secondly, most systems have a digital thermostat, these have batteries. Just pop the cover off, check in the back, you got two double A batteries. replace the batteries, put it back on the wall.

You are going to have a time delay on most thermostats, so wait 3 ~ 5 minutes to see if your system comes back on. If does you know you have solved your problem.

Next easy thing is, the drain line could be backed up. You can flush it out with a vacuum, you can blow in it out with a water hose. Either way, if your system does have a float switch, its a little device which will turn the system off its a little device which will turn the system OFF if the drain line does get backed up in to it.