check the fuses, relay and follow the wiring to see if something got cut or disconnected first (easy and cheap to fix)...if that was the problem then i flush the coolant system (you should do this evey 30k miles to be on the safe side)

see if the fan comes on when the engine gets hot (keep an eye on the thermostat)...if it doesn't work then your thermostat may be bad...you can drain the system and remove the thermostat and check it to make sure it works...there are some tests listed in the helm manual...or just plunk down the money ($30) for a new one and install it

letting the engine overheat will blow a headgasket and warp the head ($$$)