Do you mean they are spongy or they quit altogether? If spongy, you probably have very low fluid in the reservoir and you are pumping air into the system (fill the reservoir and try again - however, may need a brake bleed, which is tricky with ABS systems). If the ABS is failing, you should see error codes (using a good code reader that will read the ABS ones). Lastly, your brake master cylinder may need to be replaced.