Product review (5 stars): I fixed my own freezer!
I fixed my own freezer!
Wish I had known this info before I did it, as I would have saved about fifty bucks.

If you have a freezer that *isn't freezing*, it is often because it isn't defrosting. (The frost buildup blocks the airflow past the cooling fins.)

#1  The Defrost Timer ( Frigidaire 297318010 Defrost Timer) is VERY likely to give out eventually. It is just a mechanical timer, so it has a small motor that can go bad or its metal contacts can get fouled by the normal arcing that happens as it switches from refrigerate to defrost mode and back. This is super easy to get to and replace from the back of the refrigerator/freezer. It's about ten to fifteen bucks, so it's a cheap easy fix. (To diagnose, look for the little dial that you can turn. If you turn it, it will click and the freezer should turn off, then turn it just a little more and it should turn back on. If that happens, the contacts are fine. (If it doesn't turn the freezer off and on, then the contacts are fouled - so replace the timer.) If the contacts are fine, next mark a line on the dial and move it to the 12 o'clock position. Wait a few hours and come back and check it. If it is still in the 12 o'clock position, then the motor on the timer is dead, and you need to replace it. If however the timer has turned to, say, 2 o'clock, then the timer is still good and it is very likely that there is a different cause for the freezer to have stopped.

#2  This Thermostat ( Frigidaire 297318010 Defrost Timer ) is located inside the freezer, behind the bottom back panel, near the heat exhanger that looks like a radiator. (On mine, it was at the upper left corner of the heat exchanger.) The thermostat is fairly easy to get to, and you can replace it for about ten bucks. (Especially if it looks bloated or like it came apart at all, just replace it.) To get to it, remove the shelves, and unscrew the panel. Pull the dial for the temperature selector straight off, and unclip the temp selector from the panel. There will likely be a LOT of ice behind it, if you haven't already defrosted the unit.  If you still have to defrost it, unplug the freezer and place a fan inside of it (up on a couple boards to  keep it out of the melting water). Leave the door open and the fan going for a couple hours and the ice will all be gone, easily, except for the water all over the floor... I'd defrost it in the garage, or outside if possible.

#3  The defrost heater / heating element ( Frigidaire 216730700 Defrost Heater for Freezer ) is located under the heat exchanger, and is attached with some thin aluminum strips. It shouldn't really go bad, so I'd try replacing the above two items first. But if they don't fix it... well here's another forty or fifty bucks to throw at fixing the problem. Be careful with the aluminum strips that hold the defrost heater in place, and you can re-use them. (If they break, do NOT use a non-aluminum wire to hold it in place, or you could ruin your freezer!)

Other diagnostics:  (I haven't done these, but they came up in my searches and fixes.)
  If the fan doesn't run inside, that's a fairly obvious thing to replace, and looks to be pretty easy. Be sure to cycle the defrost timer to check it.
  If there is just not enough cooling, but there is no frost buildup and the fan works, then the temperature selector might be the problem.
  If the compressor doesn't run, there is a capacitor that I've seen come up as a replacement part.
  If nothing works at all - no lights, no fan, no compressor, then you might have had a power surge that blew out the computer board.