Confirm the thermostat
Set the system to cool, choose a temperature below the room reading and make sure the fan is not set to run continuously unless intended.
Check the outdoor unit
If the indoor fan runs but the outdoor unit is silent, note that detail. Do not open panels or touch electrical components.
Check the filter
Restricted airflow can reduce performance and may contribute to coil freezing. Replace a disposable filter only with the correct size and type.
Request diagnosis
If the basic checks do not restore cooling, a technician should test the system rather than guessing at parts.
When to request professional service
Request service when basic settings and an accessible filter do not explain the problem, or when the system is frozen, leaking, electrically unstable, unusually noisy or unable to maintain a safe indoor temperature.
Shut the system down for smoke, a persistent burning smell, repeated breaker trips, spreading water or severe mechanical noise. Call emergency services for fire, medical danger or another immediate threat to life or property.