If you're referring to the red "BRAKE" light in the cluster, it means either the parking brake is partially applied (make sure the parking brake pedal is all the way up) or the switch in the combination valve isn't centered.
When you lose one half of the brake system, the difference in front to rear pressure will push a spool valve over in the combination valve block (aka proportioning valve) and ground the circuit. A quick check is to unplug the wire from the top of the valve asy under the hood and see if the light is out.
If that's the case, most times you can get the valve to recenter by opening a bleeder screw on the half of the system that DIDN'T fail and "jab" the brake pedal sharply. You may have to do this a couple times. You want to only jab the pedal, not push it all the way to the floor, which could cause the valve to go the other way, and the light would still be on.