The most common reason an AC won’t start is a tripped circuit breaker at your electrical panel. HVAC systems have their own dedicated circuits, and when they’re overloaded or there’s an electrical fault, the breaker trips to protect your home.
Check your main electrical panel first—look for any breakers in the “off” position or sitting between on and off. Reset any tripped breakers by switching them fully off, then back on. If the breaker trips again immediately, you have an electrical problem that needs professional diagnosis.
Other common causes include a faulty thermostat, blown fuses in the outdoor unit, or a failed start capacitor. These components can fail suddenly, especially during peak cooling season when your system works hardest.