in the plastic housing for the brake sensor connector you will see another connector (i think it was blue), and that is the speed sensor connector - when you worked on the brakes, you may have pulled a wire from the speed sensor... also, the brake sensors are front left and rear right, the speed sensors are all four corners...

on a side note, when i change the brakes, since i don't trust my own skills to preserve the original brake sensor intact, i always change the brake sensor... this way, i don't have to pull the wheels again, to reinstall the brake sensor... i am just too lazy... lol...