Dirty Air Filter
If the air filter becomes clogged with dirt, it restricts airflow over the evaporator coils. If the airflow is reduced, the evaporator coils get too cold, which then causes the humidity in the room to condense on the coils and freeze. As frost accumulates on the coils, the frost restricts the airflow even more. Eventually, the entire evaporator coil assembly will frost over, causing the airflow to be completely restricted. If the air filter is dirty, replace it.