Check the circuit breaker in the breaker box. Many people still call it a fuse box, but they haven’t used fuses in years.
If the breaker to your HVAC unit has been tripped, reset the breaker. However, keep in mind that circuit breakers trip for safety reasons and if the breaker trips again, an electrician or AC service professional should look into the problem. Often, the breaker isn’t the problem, it’s something in the wiring pulling more electricity than it should.