There are many reasons for this failure. A mechanical fan has two main functions, 1, you could have a broken or slipping belt, 2 the fan clutch could be perminantly disengaged. An electric fan, well, first off, check your fuse box, as well as any relays along the circuit on the wiring diagram. There is a thermostatic switch in the radiator itself that may need replacement. Of course, there's always a bad fan motor as well. good luck man.