I should have included when you are trying to compress it with a c clamp.If you loosen the bleeder 1/4 turn will the piston move then ?
If it was solid and you were unable to move it the initial thought I would have is the caliper is frozen.
If when still under the force from the c clamp the bleeder is lossened and then suddenly the piston will now move than that indicates a problem elsewhere and that the caliper is Maybe not frozen after all. It should move when the pedal is pressed, it just won't release and will keep pressure on the pads which will then heat them up.
If you put everything back together but leave the wheel off the ground and have someone push on the brake pedal and keep pressure on it while you try to move the tire it should not move.
And after letting off the brake pedal the tire should now move freely. It the caliper is frozen it will apply clamping force but will not unclamp when the brake pedal is released.