Is the compressor running all the time except when the unit goes through a defrost cycle?
You can usually hear it but also should be able to feel it vibrating.

If yes then remove the cover in the freezer and inspect the evaporator coils.
They should have a fairly even coating of frost on them.

If they are heavily iced/frosted over you have a defrost problem.
Since you replaced the timer it is either the defrost thermostat or the defrost heater.

If there is frost in only one area then it is probably a sealed system problem and you will ned a pro. Often it makes more sense to put the dollars towards a new unit.

Since the compressor runs and you get no cooling at all, it is a sealed system problem.

This will be an expensive repair and usually it makes more sense to put the dollars towards a new unit.