The copper pipe on the outdoor unit may have a thick coating of ice around it, or you may discover that the indoor unit is totally wet, because the evaporator coil has become a solid chunk of ice.
What it the cause of an ac freezing up?
A low level of refrigerant or Freon, is usually the main cause for an ac unit frozen outside, but this is not so in all cases. If insufficient air is being moved through the duct system, this can also result in freezing.
A dirty filter, an under-sized duct or a dirty indoor coil, also referred to as an A-Coil, could be the reason for the low airflow. Setting the thermostat at a level that is too low, can also result in a frozen system. What level is too low? Each house varies, with some units even freezing up at night when they are set at 68 degrees.