Let's start by diagnosing the freezer;
Fan problem is one of the most common reasons for the freezer to start failing, this is called "frost free failure." Ice can build up on the evaporator coils over time, and this ice builds up until it starts blocking the fan. The fan is responsible for circulating cold air throughout the freezer, so if it is blocked the freezer will not be able to freeze any food. You can usually check this by removing an access panel in the back of the freezer to inspect the coils. If ice is built up on the coils, a hairdryer can assist in defrosting them.
It's also possible that the fan is not running at all. If the fan is burned out or the motor isn't running, cold air can't circulate through the freezer and everything will melt. You can normally hear the fan motor switch on and off from time to time. Listen for it and if you can't hear it coming on for an extended period of time, the motor may need repair or replacement. The wires connecting to the fan may be the problem, preventing the fan from receiving electricity.
Several other problems could be keeping your freezer from freezing:
There could be leaks in one or several of the hoses in the refrigerator, which results in the cold air being lost instead of circulating properly throughout the fridge and freezer.
If you have the freezer so full that the cold air can't circulate, your food will thaw even though the freezer isn't malfunctioning. Always leave some space open in the freezer and try to not store food right up against the cooling vents.
The thermostat might have shorted out, which means the freezer can't accurately determine how cold it is. If this is the case, the refrigerator would probably not be working, either.
If your defrost drain becomes clogged, the water in the line will just refreeze over and over again, which could damage the lines as well as the fan. Always check the drain and make sure no sludge is building up in it if you have a self-defrosting refrigerator.
Always try to keep the freezer door closed for long periods of time. The more the door is opened, the more warm air is let in and cold air is released. If the door is opened frequently, the freezer could be warming faster than the fridge can cool it, which will thaw your food........