why cant i open the doors even though all the mechaninal conections work

You'll need to remove the door panels and see what's going on inside the doors. A 2000 model GM has mechanical locks and handles. There's rods going from the handles to the latches and from the lock levers to the latches. There's plastic clips that hold the rods onto the handles and lock levers inside the doors. These clips can break and allow the rods to fall off. Once the rods are hooked up again the handles are locks will work again. Hope that helps! Jim