Assuming that you intend to place a call for a service technician, you should immediately turn off your unit. By doing this, you may prevent further damage to your system, or at least expedite repairs when the service technician arrives.
For example, air conditioning units can "freeze up", forming large amounts of ice on the cooling coil. This ice must thaw before any repairs can be made. In winter, units can overheat, requiring a "cool-down" period of up to an hour before repairs can be made.
Systems commonly fail when air filters are not changed regularly. This one problem is responsible for many unnecessary service calls. Because the air filter's function is to trap dust and impurities in your interior environment, it must be replaced often to work properly. The filter is typically located in your furnace or in a return air filter grille. You should do this as frequently once a month. Not changing filters leads to improper air circulation, which makes the unit run at less-than-peak efficiency (which means higher fuel bills), and doing so for a long period of time can dramatically shorten the life of your central heat/air conditioning unit.
A close second to the filter issue is a tripped (or blown) breaker or fuse. Many times, simply resetting the circuit breaker will correct the problem and your system will immediately resume working. But sometimes, a tripped breaker, especially if it occurs with any regularity, can indicate other problems. If this occurs during an electrical storm, or during a time of peak electrical usage , it's possible that the problem is not symptomatic of anything serious. You should allow your system to remain off for a few minutes, in order to allow the condition to stabilize itself. Then, find the breaker panel (or in older homes, possibly the fuse box) and reset the breaker (or replace any blown fuses) that supports your air conditioning system.
To reset the breaker, first push the switch to the off position and then back to the on position. If a breaker will not reset, or trips again immediately after resetting, you will need to call a qualified service company. You could possibly have an electrical short or some other potentially serious service problem.