youll probably find many many idiots have arsed around with the cable tension nuts on the handbrake rod.
Strip em down like mat said, use a wires brush on the sliding surfaces.
With the pads out pop the cable off the arm then activate the arm on the caliper with a screwdriver, if its stiff or solid then it needs freed off, pop the dust cover of and get a look, as long as theres no cracks on the caliper then lube, hammer and drift will get it shifting again.
Screw the caliper piston back in then refit it all. Pump the pedal a few times to get the pad back into contact with the disc, then go spin the disc then use the screwdriver on the handbrake arm, it shoudl lock the disc up solid with no much pressure.
Check the cables as muppets normally nip it up a bit to make the handbrake feel better. With the cable still off the caliper go pull the handbrake, make sure it pulls the cable in, they can and do sieze solid. A new cable is cheap but you can free off an old cable if you keep at it.