Start with the symptom
An AC that is not cooling can fail in several different ways. Warm supply air, no startup, freezing, weak airflow, and constant running are different clues and should not automatically be treated as the same problem.
From troubleshooting to service
Basic thermostat, breaker, filter, and airflow checks are reasonable. Refrigerant, electrical, motor, control, and sealed-system diagnosis should be handled professionally.
Plan beyond the immediate repair
If cooling problems repeat or the repair is significant, compare the repair with replacement rather than making the decision on one factor alone.
Safety note: If you smell gas, see smoke or fire, suspect carbon monoxide, or believe there is an immediate safety danger, leave the area as appropriate and contact the relevant emergency or utility service. Do not treat an active safety emergency as a routine HVAC appointment.