Your system may be low on gas, this can cause low pressure pipe to freeze, there is actually a sensor on the condenser (the cold bit in the car) to drop the aircon clutch and cooling fan power if it starts to freeze up. There is also an over temperature cut out on the compressor to protect it. Also aircon will not engage clutch if ambient temp less than 4degC.
Lack of airflow over the condenser can also cause system pipe to freeze, has the cabin air filter been replaced recently ? Is there any way moisture could have got into system ?