I agree with OUTHOUSE. The same thing happened to me a week ago when I changed Ford rotors for aftermarket rotors. The air gap between the ring on ther rotor and the fixed sensor is C-R-I-T-I-C-A-L. If it is too close, the ring will gouge the sensor, and if the ring is too far away, the sensor will not send a signal to the ABS system. This will cause an ABS light. I learned the hard way not to use aftermarket rotors unless they have the same air gap as the factory ones.
The dealership wanted $84.00 just to diagnose. I needed the brake fluid to be flushed, and the diagnosis would have revealed that the sensors were good, and the fault was in the rotors anyway. In short, a lot of money for diagnosis, and still more to allow them to repair everything.
In summary, if you have done, (or HAD DONE) any brake work involving the rotors, I would check that and the sensors again.