Nothing happens at all.

A completely dead system is less alarming than it feels. Most of the causes are protective devices doing their job, and four of the five cost nothing to check.

1. The thermostat

Is the screen on? If it takes batteries, change them before anything else. If the screen is lit, confirm the mode is COOL and the setpoint is below the room temperature, which sounds obvious and is worth ten seconds.

If the screen is dead on a hardwired thermostat, do not assume the thermostat failed. The 24 volt control circuit feeding it runs through several safety devices, and any one of them opening kills it.

2. The breaker

Open the panel and look for the AC breaker. A tripped one often does not look fully off, it sits between positions. Push it firmly to OFF, then back to ON.

If it holds, note the date and move on. If it trips again, stop. Read the breaker guide instead of resetting it a third time.

3. The disconnect outside

The disconnect is the small box on the wall near the outdoor unit. Pull-outs get removed for service and occasionally do not go back in, and their contacts corrode. Make sure it is seated.

4. The drain pan, which is the one people miss

This is the most common cause of a dead system with no other symptom in a Valley house.

Your system has a float switch in the drain pan or spliced into the condensate line. When the drain plugs, water backs up, the float rises, and it opens the control circuit deliberately. The system shuts down. Nothing else looks wrong because nothing else is wrong.

Go and look at the pan. If there is standing water, that is your answer, and the drain line guide covers what to do. Do not bypass the switch to get cooling back. Attic units sit over ceilings.

5. The float switch itself

They fail stuck open, which gives you a dead system and a perfectly clear drain. That is a flat price on the list.

What is left

If all five check out, what remains is the low voltage side: a failed transformer, a control board, or a break in the thermostat wiring. Those are a diagnostic, and the price is published.

Before you call

Have ready: whether the thermostat screen is lit, whether the breaker has tripped, whether there is water in the drain pan, and the brand off the outdoor unit. Those four answers cut the phone call in half, and often mean the fix gets identified before anybody drives out.