could be many things really.

Have you done any work on the brakes recently or any work in the area?
if so you may of disturbed a sensor or cable!

Make sure  your wheel sensors are fully home.
Then check the cables from the sensors, these could be broken, worn away,
snagged, unplugged etc

Then if you dont find anything you'll need to jack up the disco and put a
meter across each sensor in turn to see if its working!  If i remeber right
you set it to A.C. volts @a low range spin the wheel and you should get a
value.

If not then thats your problem sensor. remove it and check its face for
damange, if all looks good clean the hole and slide it back in (may need a
replacement O-ring). then retest.

If still no joy then a trip to a dealer or specilist with a fault code
reader to pin point your problem.

best of luck

Jinx