rear caliper seized?

[Goran]

I tried to replace my rear discs and pads today. Unfortunately I was stopped in my tracks by a caliper that wont rewind back in. also the boot has come off, and has been off for some time judging by how dry the piston is underneath, almost no grease at all.
Is this a completely done-for caliper or could it be that the handbrake cable has seized and wont let the piston rewind? Not sure why it wont rewind back in.
I left the handbrake off throughout, although I let the car roll back on the front brakes when in park, would this have applied the rear brakes too?

I forgot to add, another bit of evidence is that the pads on this caliper are almost gone, down to the backing plate, whilst the pads on the other side are still good, maybe more than 10mm left. I guess this is a sign that it has been seized for a while now?