I assume you have an ABS warning light illuminated on the instrument panel.
If so, a fault code will be logged in the ABS electronic control unit (
). You need to get this fault code read by a dealer, or independant garage specialising in vehicle diagnostics. Once we know the fault code, we can advise further.
Be aware that with the ABS warning light illuminated, you may not have hydraulic pressure regulation to the rear brakes. Under adverse braking conditions the rear wheels could lock (this function is normally controlled by the ABS system).
As a start however, have you had anything done to the car recently in the braking/suspension/steering area, or had new tyres fitted, or had a wheel removed for any reason?