Be Ready — Magnificent

Prepare

Know the make and model of your HVAC unit (written on a label on the outdoor unit and inside the air handler).
Locate your breaker panel and identify which breaker controls the AC.
Know your landlord or building manager's contact info — save it in your phone.
Save a local HVAC technician's number in your contacts.
Keep a box fan available as a backup for hot nights.

Change the air filter every 1–3 months — a clogged filter forces the system to work harder and is a leading cause of failures.
Do not set the thermostat below 78°F when outdoor temperatures are above 95°F — the extreme differential overworks the compressor and can cause capacitor failure.
Schedule annual HVAC maintenance (spring, before summer heat) — a technician can catch failing capacitors before they fail completely.
Keep the outdoor condenser unit clear — trim vegetation at least 2 feet around it and hose it down annually.

Check the thermostat — confirm it's set to Cool and the temperature is below current room temp.
Check the breaker panel — the AC breaker may have tripped. Reset it once. If it trips again immediately, do not reset — call a technician.
Check the air filter — if clogged, replace it. A blocked filter can cause the system to freeze up.
Check the outdoor unit — is the fan running? If the unit is running but not cooling, the refrigerant may be low or the capacitor may have failed.
Turn the system off at the thermostat for 30 minutes if it may have frozen up (ice on the lines), then restart.
If none of the above resolves it: contact your landlord or an HVAC technician.
While waiting: close blinds, use fans, stay hydrated, and identify cooling centers in your area if outdoor temps are dangerous.

Your landlord or building manager — always the first call for rental properties
Search "HVAC repair near me" for licensed local technicians
Local cooling centers — search "[your city] cooling center" during heat emergencies
HVAC filter size is printed on the edge of your current filter