Most do-it-yourself homeowners can replace either a freezer fan motor or the entire freezer fan without much difficulty. The problem may just be ice buildup on the blades, but the motor or the thermostat could also have failed.
Checking on the fan starts with unplugging the refrigerator and taking off the evaporator fan cover. When blades are icy, letting the ice melt generally solves the problem. Bent blades require replacement. If the fan blades turn freely, unhook the fan wires, and use a multimeter to look for continuity through the windings. Without continuity, it's time for a new fan motor assembly. If the assembly isn't the problem, check for voltage at the test leads on the motor. If no voltage reaches the motor, a thermostat could be the issue.