First check your electrical breaker or fuse box. If you have had a recent storm or electrical surge, it is possible they have tripped the breaker or blown a fuse. Your air conditioning unit should have an outdoor fuse box located near the unit on the wall of the house. Test these fuses to ensure they are not broken.
If you notice ice on your AC coils, turn the system off immediately. Failure to do can cause damage, and repair work cannot begin until the ice is removed.