Was it a defrost timer or a defrost control board?
Was the frost pattern on the coils a even coating, see below?

I am assuming that the evaporator/freezer fan does run.
If not that may be your problem.

Remove the cover in the freezer so you can inspect the evaporator coils.

If they are frosted/iced over to the point that the fan cannot pull air through them then it is a defrost problem.
Force a defrost cycle to see if the heater comes on.
Note that the defrost thermostat must be frozen as it opens just above freezing which kills power to the defrost heater.

If there is just frost in one area (usually where the freon enters the coils) then it is most likely a sealed system problem.
This will be an expensive repair as you need a pro. In many cases it makes more sense to put the dollars towards a new unit.
Be sure that the compressor has run for at least an hour before doing this. If you do it shortly after or during a defrost cycle then you may make the wrong diagnosis.