check your fuses first.....theres one in your fuse box under the hood that controls the fan and there is a relay along side the bay right behind the headlamp, it's attached to the wall of the engine bay.

If thsoe are fine, it might be your thermo switch which is on the back of your engine (beside your oil filter).

My thermo switch went bad and it's such a biatch to get to, I just pulled off the two wires and wired them to a toggle switch which I put up under my dash. so I can turn on/off my fan at will. I usually just keep it running all of the time though. the switch will just complete the circuit. Very easy to do.

If it's not the thermo switch, it could be your fan motor is busted. I believe this can be checked w/a voltage meter, but don't quote me on that.