Basically could be what morgey said or: Seized handbrake cable. Seized Piston. You need to disconnect the cable from the caliper and check it is moving when the handbrake is applied and released. Then you can remove the sliders give them a light rub with some emery paper, also clean the caliper carriers in case the pads are jamming. Once you've done all this grease the pads and the sliders make sure they move freely. Dont use regular grease use either silicon or Id reccomend copper. And build everything back up. This is all free to do for now and should fix your issue. If the cables arent moving freely you need cables if they are fine and it persists after all of that then you need to either get replacement calipers or rebuild the ones you have.