A/C Unit Has Leaking, Has Ice Buildup
Inadequate refrigerant causes the evaporator (and sometimes the condensor) to form ice or frost on the outside.

The usual fix is to have a HVAC tech repair the refrigerant leaks and recharge the system.

You can save some of the service call cost by looking for the leaks and pointing them out.  Soapy water works well applied with a spray bottle or small paintbrush.