If your air conditioner stops working suddenly, first check the thermostat to ensure it's set to the correct temperature and mode (cooling). Next, inspect the circuit breaker or fuse box for any tripped breakers or blown fuses, and reset or replace them if necessary. Check the air filter and clean or replace it if it's dirty, as a clogged filter can impede airflow. Ensure that the outdoor unit is free from debris and obstructions. If these steps don't resolve the issue, turn off the system to prevent further damage and call a professional emergency cooling service for a thorough inspection and repair.