Why is my ABS light on right after I got new brakes installed on my bmw

The abs sensor reads a sprocket like gear. They are a few possible answer. One, if BMW sprocket is mount on the rotors like most cars, the new rotor could be an aftermarket without gear. Two, the sensor could need to be cleaned Or adjusted. or the sensor has been damaged. Last by not least, when they installed your new brake pafs, they would have to compress the brake calipers. I do not know the exact BMW process for this, but some cars has to be done in a special order or way. If not, you can get air in abs pump of trip the brake module to think something is wrong. My suggestion is to return back to shop and explain that to he and light was not on before. Hope this help