A model number would be useful.

Check to see if the compressor is running. Should be warm and you should be able to feel it vibrating.
If yes
Remove the cover from the evaporator coils so you can inspect them.
Check that the evaporator fan is running.
Be sure the unit has been running a couple hours before doing this. In other words do not do it just after a defrost cycle.
If they are clogged solid with ice and frost you have defrost problem.
If they only have frost in one area, usually where the freon enters the coils you are low on freon and have a leak somewhere.
If no hint of frost or cooling you are either out of freon or the compressor is toast

The latter two usually mean the unit is toast as repairing it will be expensive.

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