The answer
- Thermostat set to "ON" instead of "AUTO" (blower runs continuously when compressor is off)
- Dirty air filter choking airflow and freezing coil
- Refrigerant leak in closed loop
- Outdoor breaker tripped or failed dual-run capacitor causing outdoor unit not to run while indoor fan blows room-temp air.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.