The answer
- Restricted airflow (dirty filter, closed vents, dirty blower) prevents heat absorption, dropping coil temperature <32°F
- Low refrigerant reduces suction pressure and saturation temperature below freezing. Action: Turn AC OFF, set fan to ON to thaw. Do not run while iced to avoid liquid slugging and destroying the compressor.
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.