Its faulty compressor if freezer section is also not cooling.
But if freezer section is working fine,then problem is confirm defrost assembly.

Replacement parts can be purchased at any of the following web sites:

searspartsdirect.com
pcappliancerepair.com
appliancepartspros.com
repairclinic.com

partsselect.com

--------------------

Before going to buy any parts directly.
Go through this troubleshooting help links.

-------------------------

This is list of refrigerator problems, which is normally faced in refrigerator.
You can go through the details, by clicking the link below:---

--------------
Refrigerator is not cooling properly?
http://technoanswers.blogspot.com/2011/06/refrigerator-is-not-cooling-properly.html
-----------
Both refrigerator and freezer not cooling properly in refrigerator?
http://technoanswers.blogspot.com/2011/06/both-refrigerator-and-freezer-not.html
----------
The ice maker not working: ---
http://technoanswers.blogspot.com/2011/06/ice-maker-stopped-working.html

------------
Compressor not running on refrigerator?
http://technoanswers.blogspot.com/2011/10/compressor-not-running-on-refrigerator.html
--------
Water dispenser not working on refrigerator?
http://technoanswers.blogspot.com/2011/10/water-dispenser-not-working-on_19.html

-------------
Ice maker stopped making ice?

http://alltypesofpartsreplacing.blogspot.com/2011/10/ice-maker-stopped-making-ice.html

-------------
Water Overflows in Ice maker and freezes inside?

http://alltypesofpartsreplacing.blogspot.com/2011/10/water-overflows-in-ice-maker-and.html

-------------
How to replace Ice maker assembly?

http://alltypesofpartsreplacing.blogspot.com/2011/10/how-to-replace-ice-maker-assembly.html

These will help.

Thanks,.