This is not rocket surgery. Here's a simple guide that may save you big bucks. First check the obvious - popped breakers, meteorite strikes, dead rodents on power lines. Reset the breakers, call NASA, or call your power company.
If the breakers pop quickly when reset, move to the MV section, disregarding the LV part. If you have a frankenstein type switch with fuses inside, likewise.
The Low Voltage Part.
If the AC is not working but the breakers aren't blowing, yo most likely have a Control Circuit problem. The Control Circuit is run by the thermostat
Does the Thermostat do anything? By "anything" I mean make the fan turn on and off. Set the Thermostat to "Fan Only" If no, Take the dust cover off the thermostat and blow out the dust using the canned air you use on your computer keyboard, or just vacuum it out and try again. Works now? Yay!
NO? You wouldn't have such a thing as a voltmeter in the house, would you? You should.
(A brief digression on voltmeters. You can get a non-contact volt sensor that registers line current for about $10. For lower voltage Direct Current, you can get a thing that looks like a small screwdriver for about $4.00. You can get a cheap multimeter for $10. Don't. I use a $4 dc volt probe, a $10 non contact sensor, and a $250 Volt Ohm Amp meter. Just remember to turn the voltage sensor off when you're finished. Replacement batteries cost more than the tool itself)
But anyway, your problem is probably the Transformer which supplies electrons to the Thermostat, which directs them to switches in the fan motor, AC compressor, and Heater elements. If there's been a lightning strike nearby, or if you've had a service outage, this is the likely culprit. Attire yourself appropriately, find a couple of flashlights, a set of needle-nose pliers and a screwdriver (if you've got one of those flip flop ones where you can turn the bits around and turn the shaft end to end you're ahead of the game). Follow the wire that looks like a telephone line. This is a good time to confirm that you're working on an unpowered circuit. You may have to remove a side panel. On one side of the transformer will be a black and a white wire (if you're lucky). They'll come from one side, and be bigger wires. On the other side will be poor anorexic wires, probably wire nutted to wires of different colors. Don't judge them, just use the tech to take a picture so you know what goes with what, because the transformer output color coding is set in stone and the contractors used what they had. Pull off the spade connectors with the needle nose pliers and tape the to one side.
Take it to your local HVAC or electronics shop (I am lucky - I have Johnson Supply about a mile and a half away) and replace it with the same or upgrade for generally under $15.