Problem: no air
If you switch on the central unit and nothing happens, the problem could be as simple as a blown fuse. Check your home's fuse box and reset any blown circuit breakers.
Try setting your thermostat to its lowest setting. If the unit still doesn't turn on, your thermostat might be broken and will need to be replaced.
No power can also be a sign of broken or faulty wiring. Call an electrician or heating and cooling specialist to inspect your installed system.
Problem: warm air
If your air conditioner is blowing air but it isn't cool, the problem could be due to a blocked air filter. Check your cooling system's manual for advice on how to clean or replace the air filter.
Otherwise, the problem might be a lack of refrigerant. Call an air conditioning contractor to replace the coolant and check for leaks in the refrigerant system.
A frozen air conditioner is another cause of warm air. This can happen during extended periods of use. Open the unit and check if any ice has formed inside. If so, turn the power off and let your system defrost for a few hours.