If your air conditioner won't turn on, try the following before hiring an HVAC specialist:
Make sure the outside breaker fuse is in the right place.
Look for a tripped breaker in your electrical panel.
Check for a blocked air filter and make sure your thermostat is turned on.
Make sure your condensate pump is working properly.
If you verify them and everything appears to be in order, you will need to call an HVAC expert for repair service.