Check your fuse box and make sure that your breaker is not flipped. If the breaker has been tripped, then no power will go to your system. Once you have switched it back on, check that there is power going to your thermostat, air handle, and outdoor HVAC unit. If you notice your breakers tripping regularly, then you may need to contact a qualified electrician.
The contactor (relay) and start/run capacitor(s) (see illustration below) fail most often and are inexpensive. So it’s a safe bet to buy and install those parts right away, especially if your air conditioning service unit is older than five years. The condenser fan motor can also fail, but it runs about $150 — hold off buying that unless you’re sure that’s the culprit.
"Very impressed. Returned call within 24 hours. They came out the same day I spoke with them and made the repair within 25 minutes. Very low service call fee. I had another repair service out two weeks earlier and after 3 service calls and the central air still not fixed and temperatures in the high 90's, it was such a relief to have a company identify the problem, have the necessary part with them and fix the problem."