(I'm going to assume firstly that the caliper is free from the hub, that the handbrake is off and you've got the cap off the brake fluid reservoir).
It sounds to me like the piston is very tight in the bore, hence why it wont turn. Usually thats new caliper time. However, sometimes the flexible hose can break down and prevent fluid movement. Next try cracking the bleed nipple open and see if the piston retracts more easily. If it doesn't you're back to replacing the caliper, if it does, try replacing the flexible hose.
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