This is usually caused by the temperature switch failing which is fitted to the bottom of the radiator on the left hand side where the coolant hoses are. Check this is your problem by removing the connector wire to the fan switch and shorting the two pins together. Switch the ignition on to test if the fans start to work. Fixed by simply replacing the switch with a new one. You can use this as a good time to replace your coolant (by drain and refill and bleed) or simply unscrew the old switch and replace with the new switch and try not loose any coolant. You might introduce some air pocket but it will usually work it way through the system no problem.