Both the ABS and ATC rely on wheel speed sensors to work properly. Odds are one or more wheel sensors have failed, or, more likely, had a wire come loose or corrode.
As Tony Di says, the ABS light means the anti-lock function isn't operative. The brakes still work fine, but they now rely on the driver to detect a brake lock-up and adjust accordingly. ATC light means the Automatic Traction Control isn't working, so you might spin a rear wheel on a slippery surface.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL