The short answer
An AC that will not start at all is a power or control problem before it is a big problem. The usual causes are the thermostat, a tripped breaker or disconnect, a float switch shutting the system down over a clogged drain, or the control board. It is often a small repair.
What you can check yourself
- Check the thermostat: Cool, a set point below the room temperature, and batteries if it takes them.
- Check the breaker for the air handler and for the outdoor unit. Reset it once at most.
- Look at the drain line and the pan for water. A full pan can shut the system off on purpose.
- Make sure the service switch near the indoor unit is on.
What gets checked first
- Thermostat wiring and the sub-base
- Breaker and the disconnect
- Float switch and the drain line
- Control board
Repair or replace?
Usually a repair, often a small one.
When to call now
If the breaker trips again right after you reset it, leave it off and call.