2000 Buick Century no brakes while driving

If you had to put fluid in that means that it has leaked out somewhere. Leaks can occur anywhere in the brake system. However, some of the most likely places are the wheel cylinder (drum brake--rear), rear of master cylinder, rubber brake lines (at each wheel), and the calipers (disc brakes--front). Rusted brake lines can also allow fluid to leak. Have all of these possibilities checked by a certified technician. Post any other questions. Also, once the problem is solved, please post the cause and solution.