The coil in the furnace is froze up. Turn off the unit outside and turn the fan on in the furnace. Let it run to thaw out the coil. Probably get some water around the furnace. After it si all thawed out, check the filter and make sure the fan is running at high speed. If those are OK, you are probably low on refrigerant (R-22 probably) and need to add some. Have you ever had to add refrigerant to the system?? Also, make sure the outside coil is clean. Wash it out with water to make sure the coils are clean! Hope this helps! Been a commercial HVAC Tech and Plumber for 25 years. IF you have any more questions, let me know.
