There may be many causes. The first to check is the Power Relays located in the engine compartment, on the Battery-side. There is a plastic cover that can be seen with drawings on it related to the relays. Concerning to Cooling fans, there are 5 of them. Three for normal cooling operation, which are:
1. Rad LO I, for low speed of radiator fan, the one closer to the battery and operated by one of the two thermostats.
2. Rad HI, for hi speed operated by the second thermostat and A/C Control unit
3. Rad LO II, for low speed of Rad fan, this time operated by the A/C control Unit, anytime the A/C compressor is working, cooling is activated
4. Cond LO, for low speed of Condensator fan, the one located by the Rad fan side, and operated as the previous one (3)
5. Cond HI, for hi speed of Condensator fan, also operated by both thermostat and A/C control unit.
If any of these relays won't work, there is still a chance for one of the rest to do. You should tell which function is not operating. If the A/C operates, does both fans operate at low speed? if not, check the related realys.
If at normal operating temperature, both thermostats control the two speeds for fans. At first speed, only the Rad Fan runs and the second speed (Hi) both fans run.
Well, after checking the relays, if no bad relay appears to be, then you should check the thermostats located bottom of the radiator case, just under the Rad fan. If you "bridge" the connector of those thermostats with ignition key in ON position, once the connector of any thermostat has been taken out, the fan should run
(one or two depending on which thermostat is chosen).
So it is dealing with relays and theremostats. If both appear to be good, there should be a bad-contact in the related connectors, before you can tell the fan motor is broken. It can be also checked by using a multi-tester for resistance and volts, but explanations come to be longer.