Direct Answer: This is usually caused by a clogged air filter, a tripped breaker, or a frozen evaporator coil. First, check your filter—if it's gray and fuzzy, replace it! If that’s not it, check your outdoor unit for debris. Honesty Check: If you see ice on the lines, turn the system off immediately and call us. Running a "frozen" unit can burn out your compressor, which is a much more expensive fix than a simple refrigerant recharge or leak repair.