When your ABS light comes on, it’s probably because something transient caused the system to go haywire. It might have been a bad tire or a piece of debris from your last drive through a snowbank. The first thing you want to do is check the main wiring harness for any damage. You might need to unplug and re-insert it to see if it helps.

If that doesn’t work, you’ll need to look at the wheel speed sensors. The ABS controller relies on wheel speed signals from each of these sensors, which are located on the axles and in some cases the wheel bearing assemblies themselves. There are several different types of sensor, but they all have a toothed wheel and a magnetic pickup. The tone ring assembly is where the magnets pick up the data, and it can be damaged by things like rocks and other foreign objects.
Some shop technicians can use an ABS code reader to access trouble codes stored in the controller. They typically charge a modest service fee.
If all else fails, you might need to replace the entire ABS unit. Unfortunately, that’s going to be pretty expensive. If you can’t afford to do that, you’ll need to take it to the dealership or a mechanic who works on this type of system. The problem with the ABS controller is that it’s a complicated, hard-to-service combination of hydraulics and electronics that doesn’t have user-serviceable parts inside.