Firstly - check your coolant level; have you any leaks? Or do your have mayonnaise-like substance in your oil (coolant and oil mixing)?
If that looks ok, check your fan fuse (has it blown?), fan motor (if you can trace the wiring, try and run it straight off the battery, if it runs its ok), fan relay (with ign OFF, brown / black wire should have voltage, red / white wire should have voltage with ign ON). If thats ok, unplug the relay connector, and using a multimeter on ohms, go between the relay brown / black wire and the blue wire (it should show as an "open circuit", ie no continuity). If thats ok, then put 12v (battery) positive (+) to red / white wire and battery negative (-) to the green / yellow terminal - the relay should click (you'll hear it), and your meter should show 0 (zero) ohms (if it does, the relay works). If the relay shows no reading all the time (ie your fan never works), or 0 ohms all the time (fan always on), the relay is knackered.
If the fan and relay are both ok, check the coolant temperature sensor: place it in water / coolant with a thermometer, and you should get the following readings: at 0C - 5.21 - 6.37 K ohms, 10c - about 4K ohm, 20c - about 3K ohm, 80c - 290ohm-350ohm.
If you aren't sure about electrical testing / don't have access to a multimeter, then you can try substituting the relay / sensor for a known good one and see if your fan works.
On the S2, the sensor supplies a signal to the ECU, and the ECU tells the fan relay to come on.
Hope that helps.