1987 300SDL abs light on

My abs warning light came on last night after a hard stop in the rain-this happened shortly after I ran over a large tree branch which broke up and struck the underside of my vehicle in various places-the brakes still feel as if operating normally and stop the vehicle just as before the light came on- any suggestions as to where to start? I have ordered Kent's brake manual but it will be several days before I get it-Thanks!