You need to remove the wheel(if not off already), and have someone press the brake pedal down while you watch the caliper.
If the piston does not extend, then you have a collapsed hose.
If it does, then observe the caliper slides to see if the caliper travels on them. If not, then remove, use emory cloth on them, lube with brake lube and reinstall.
If they do, then remove the caliper and manually try to move the pads to see if they are sticking. If they are, they may bee a poor cut on them causing too tight a fit and not allowing them to move along the caliper rail. Use a dremel or grinder to remove a little material until the slide freely and do not hang up
Most of the time it is either the caliper pins or the pads hanging up and not moving freely
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