How to reset AC contactor?

You cannot physically "reset" a standard residential air conditioning contactor with a button or switch once it has failed electrically or mechanically, because contactors are electro-mechanical switches made of consumable copper points and wire coils. If the contactor's mechanical plunger becomes temporarily jammed due to trapped insect debris or minor dirt accumulation, you can carefully clean the debris out after turning off all electrical power at the main circuit breaker and outdoor disconnect box. However, if the contact points are severely pitted, burnt, or welded shut, or if the internal electromagnetic coil is burnt out, the contactor cannot be reset and must be replaced entirely with a new unit matched to your system's voltage and amperage specifications. You can verify that your corrective action was successful when turning power back on allows the contactor to engage cleanly and quietly without buzzing or staying stuck.