The Anti-lock Brake Computer detected a problem, set a diagnostic fault code, turned the system off, and turned the yellow warning light on to tell you. If the red warning light is also on, that means there's a problem with the brake fluid level, a leak in half of the system, or the parking brake is not fully released. With those conditions the ABS Computer knows it may not be able to operate properly so it shuts off. You may very well not have an ABS problem. First solve the red light, then restart the engine. If just the yellow light turns back on, have the diagnostic fault code read to see which circuit or system needs further diagnosis.