The switch has flipped itself reversed.. the thing is supposed to turn on every 8 hours... this one turns 0ff ever 8 hours.. Somehow the off/on switch fell out of correct cycle..
with the defrost running almost full time, the compressor has to work extra duty too..
A mech defrost timer can fail three ways.
1 It gets stuck in defrost. Compressor is off and in most models so are both fans, nothing runs so no cooling. Turn the knob, if the refer starts replace the timer.
2 It is stuck and won't go into defrost. You won't notice this for 10 or 12 days. Since it won't defrost, frost builds on the evaporator so thick the fan can't move any air thru the coils, so no cooling. If the back panel in the freezer has a thick layer of frost on it- it's not defrosting. This could also be caused by a failed bimetal (defrost thermostat) or an open heater. The bimetal is a 1" diameter can clamped the the evap coil, has 2 wires.
3 The switch contacts break/burn it's the least likely. Shake it if rattles badly the points are broken off-replace the timer.
First thing to do is check that both fans are running, then check to see if the comp is running.
If they all are pull the evap cover in the freezer. You should have a complete frost pattern. If not and every thing else is running you have a freon leak or restriction-go buy a new refer.
If the evap is packed with frost-it's not defrosting. Turn the knob on the timer CW until it clicks and the fans /comp stop. Check the heater it should be getting hot. If not then the heater or bimetal is open. Most likely the bimetal, heaters seldom fail, unless they the glass tube type. Check the plugs on the heater and bimetal connections, sometimes ice gets in them and forces the connector apart.
If the heater does come on the the bimetal/heater/wiring is good and you need to replace the timer.