Put the pads back into the caliper, and using a plastic tyre leaver to push the pistons back! And then refit and try. If still fault – Are the pads worn? How old is the brake? Do you use for downhill riding? I would next bleed the brake. If pads are worn and brake is old, replace with SLX or XT From riding down hill, pads /rotor generate a lot of heat. Also check the rotor for being over heated blue in colour and also replace.