Checked behind the back panel of the freezer. The coils were covered with ice, Then went to the top of the refrigerator, under the light fixture. Found the timer and checked to see if it was advancing. It was stuck. Manually advanced the timer to the defrost cycle. Waited for a few minutes, but the heating element in the the freezer did not warm up. Looked at the defrost thermostat; it's top was pushed up. Decided to replace the timer and the defrost thermostat. This solved the problem of no defrost.
When everything was put back together, the refrigerator got too cold. Could not adjust the refrigerator compartment temperature. Found that we had not correctly connected the belt that controls the damper door. Readjusted the belt, everything is fine now, the freezer is at zero degrees, the refrigerator is at 40 degrees.
The most challenging part of this repair was testing the defrost thermostat. I tried testing the thermostat in ice water, but as I understand it, the thermostat comes on below 30 degrees, so testing it in ice water isn't accurate. It measured continuity in this test. I also tried testing the thermostat while it was connected, by closing the freezer and waiting for it to cool down, then testing for continuity between the thermostat and the heating element. The problem with this is that, in order to make the test, I had to open the freezer door, which allowed the freezer to start warming up. For some reason this test also measured continuity, even tho the thermostat was defective. It could be that it had a short that made the continuity. The top being pushed up was the only sure indicator of a bad thermostat that I went by..
Afterward, I thought that one way to test the defrost thermostat may be to use dry ice.