Car Brakes

I took my car in for new front brakes and the garage put them on. When I paid the bill and left I drove around doing my errands and when I put the brakes on a little harder the brake lights came on. The big brake light and the ABS light both came on. Then I took it back to the garage and complained about it. The mechanic hooked up the computer and said we found the problem but can't fix it today and it will probably be Monday before we get it done. I told him it never bothered before until you fixed the brakes and if that's the case I'm not paying for further work. I'll just drive it like it is until it falls apart. He unhooked the computer and I left. I drove 12 miles making numerous stops and the lights never came on. Even with many shutting off the engine several times and restarting it still nothing. I just don't know what to think.