1. Check the Thermostat Settings
It sounds simple, but ensure your thermostat is set to COOL and the fan is set to AUTO (not ON). If the fan is set to ON, it will blow unconditioned air even when the compressor outside isn't running.
2. Inspect the Air Filter
A severely clogged air filter restricts airflow across the evaporator coil, causing the system to blow weak, relatively warm air. Pull the filter out and hold it up to a light. If you cannot see light through it, replace it immediately.
3. Check the Outdoor Unit (Condenser)
Go outside and look at the condenser unit. Is it making a humming noise? Is the fan spinning? If the indoor unit is blowing air but the outdoor unit isn't running at all, check your main electrical panel for a tripped breaker explicitly marked "AC" or "HVAC".
Still blowing warm air?
If the filter is clean and the breakers are fine, you most likely have a failed capacitor or a refrigerant leak. Under EPA Section 608 regulations, handling refrigerant requires a licensed professional. It is time to dispatch a technician.