I cannot find any info on the given model number.
Please check it.
Could it be a SBD20KW?

Is the compressor running all the time except when it goes through a defrost cycle?
Or is it cycling normally?

Is the evaporator/freezer fan running?

If on all the time remove the cover in the freezer so you can see the evaporator coils.

If they are heavily iced/frosted over you have a defrost problem.

If there is no frost or frost only in one area (usually where the freon enters the coils), then 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.
Since the above could result in scrapping the unit be sure that the compressor has run for at least an hour. Checking during or shortly after a defrost cycle could result in coming to the wrong conclusion.