the defrost fan located at the back of the freezer is probably blocked by ice.
this is caused by the dfrost unit being faulty.
i have had this happen to me.
You could try a manual defrost (to melt the ice) But you may need to get a defrost system fixed.
This is not likely to be to expensive and could be a 5-10 min job.
see http://www.applianceaid.com/frig_notcold.html
nick schlensky, December 2007