Here are your parts
Replacement parts for MSD2556AE models | AppliancePartsPros.com

Here is a service manual
http://www.servicematters.com/m...ibrary/docs/16010154.pdf

We have replaced the compressor on this unit three times in the past, latest being around 3 years ago. The freezer was never a problem in the past until now.
You must be one of the most optimistic people in the world, your cup must be always 3/4's full.
If I had replaced a compressor 3 times I definitely would not say "The freezer was never a problem in the past".
Quite frankly after the first time it would have ended up in the garbage.

First ensure that the compressor is running.
If it is not and the evaporator and compressor fans are also not running you may be stuck in a defrost cycle. Depends on how long it has been sitting as to how cold the freezer is.
If just the compressor is not running, it may be a thermostat, start relay or overload problem.

If it is running then check that the fan in the freezer compartment is running.
Remove the evaporator cover so you can see if the coils are heavily iced/frosted up. If they are you probably have a defrost problem.

Turn the defrost timer till the compressor, it's fan and the evaporator fan shut off. You are now in defrost.
Does the defrost heater come on?
Be careful that you do not burn your fingers.
Note that the defrost thermostat must be frozen, it opens just above freezing.
If it defrosts then your defrost timer should be replaced.

If it does not then the defrost thermostat or the defrost heat may be toast.
Note: this does not totally rule out the defrost timer as it's contacts may be shot. See the manual and the links below.

If the evaporator coil just has frost on a area where the freon enters it from the compressor you are probably low on freon.

Here are a couple good sites on basic fridge troubleshooting/repair
http://www.applianceaid.com/refrigerator.html
Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com