There are 2 removable brake pads. You could see if one or both of the pads has wiggle room in it's seat. Each pad had a metal handle that you can grasp with a pair of pliers to remove the pad. There is a spring that holds the pads in place. If that's not it, then I'd guess you might have to replace the whole brake. Is it hydraulic or cable-activated? I've never known anyone to disassemble and repair a disk brake caliper, and parts may be hard to find, but maybe it's possible.