The vents are blowing warm air.

Warm air at the vents is a different problem from no air at the vents. Air is moving, so the blower is fine. Something between the refrigerant and the outdoors is not.

The free one, first

Check that the thermostat fan is set to AUTO, not ON.

On ON, the indoor blower runs continuously, including during the long stretches when the compressor is off. You get a steady stream of room-temperature air out of every vent, the house feels worse, and nothing is broken. This comes up constantly and it costs nothing to fix.

While you are there: confirm the mode is COOL rather than HEAT or FAN, and that nobody has set a schedule or a hold that is doing something unexpected. Smart thermostats run away with this.

Then go outside

Is the condenser running? Stand next to the outdoor unit. You should hear the compressor and see the fan on top spinning.

If it is completely silent while the indoor blower runs, the problem is on the outdoor side. Check the breaker in the panel, and check the disconnect box beside the unit. If both are on and nothing is happening, the usual suspects are the capacitor and the contactor.

Is the fan spinning but the compressor silent? That is a specific and useful observation. It usually points at the compressor side of a dual capacitor, or at the compressor itself.

Is the unit running but the fins packed with dust? A fouled condenser coil cannot reject heat. The system runs all day and delivers very little, and on bad afternoons it trips itself off and restarts.

The ones you cannot see from the driveway

Low refrigerant. A sealed system does not use refrigerant up, so low means a leak. Low charge gives poor cooling and often frost somewhere.

A failing compressor. The outdoor unit hums and runs but moves very little heat.

A reversing valve, if you have a heat pump. Stuck in the wrong position it will heat your house in July while every other component works perfectly.

What finding out costs

If the system is running but running badly, that is a diagnostic: pressures, amp draws, capacitor microfarads, temperature split, and photos. You keep the readings whether the work gets done here or not.

If it turns out to be a capacitor or a contactor, both are flat prices already on the list, and you hear the number before anything is touched.

When to stop

Turn it off if you see ice on the copper lines, if the breaker trips a second time, or if anything smells like burning. None of those improve by running the system harder.