It could be a wide variety of problems from a failure in the wiring, the abs modulator unit to one of the front or rear abs sensors (they are easily damaged removing a wheel or even a central computer failure but if everything else is fine that should be OK. One of the easiest ways to check is to get a dealer to plug her in and see what fault codes come up. It's probably the best way as you could spend ages replacing the parts and murphys law says it will be last bit you replace that is causing the fault. Sadly, I think there are no shortcuts here. Also if the light remains on under the new MOT rules it's an automatic failure. Sorry if this is not what you want to hear.