Sounds like it is seized. Did that side wear out faster than other side? Is the other side OK? If it is seized and you want to try to rebuild it your self, take it completely off car and put it in a vise,use a piece of wood between piston and caliper,to prevent damage, take your blow gun(assuming you have air comp) blow air thru where brake hose hooks up(leave bleeder tight. See if that will dislodge piston.Be careful and do it slowly, will come out with gusto prolly.
Hope this helps.
rickf
