ABS has become pretty much standard equipment on most vehicles. Sensors tell a computer when a wheel stops rotating, which indicates at least when the vehicle still has forward speed that the brakes have overpowered the available traction at that particular wheel. The computer then directs a hydraulic valve to release some brake fluid pressure to the wheel to let it rotate again. This process repeats many times per second until the vehicle stops or you lift your foot off the brake pedal. The ABS computer does a power-on self test every time you cycle the ignition. If it finds it's lacking data, or a hydraulic pump or valve isn't responding, it illuminates the ABS warning light on the dash. ABS relies on a properly operating conventional brake system. If the ABS packs up, you should still have normal, unassisted braking, so it's safe to continue your journey.
There are many things that can cause the ABS light to come on. Some are serious, and therefore a diagnostic check should be done immediately. But there are times when the light comes on and can be met with a simple solution. For instance, a dirty/faulty ABS wheel sensor can cause the system to trigger the ABS light during self-evaluation.
Check the fuse for the ABS unit, the fuse may be in the fuse panel inside the passenger compartment or bonnet.
Check the service manual for the voltage and resistance values on various pins and sensors. The main harness to the ABS controller should be checked; Inspect carefully for any signs of corrosion—remember that the signals travelling down some of these wires are only millivolts and almost any resistance is a major impediment. If you can, check the resistance across the wheel speed sensors. There may be damage to the wiring leading to the ABS sensors or even the tone wheels or sensors themselves.