Rear brake piston jammed

Just replaced my front pads with ease and moved on to the rears.

Safe to say that didn't go accordingly!!!

The piston seems to be jammed out to and however much pressure, including a vice grip it just will not budge, what could this be due to? Car is in a garage safe and sound up on axel stands but want to get to the bottom of this as soon as possible!

quick suggestions please, I'll be up all night on it ha?

Thanks
 
you have to wind it in - clockwise whilst applying pressure

oh and make sure the handbrake lever is fully off/back