rear brake caliper piston stuck

[Shifu]

If they worked fine then it is unlikely that they are stuck all of the sudden. It is possible that, if the pads are used all the way to the backing plates and your disc is a bit thin, they could jam because they almost all the way out of caliper. Especially if you have used the implements like you described to push them back. It is likely they are on the piss and will not badge untill you get them straight again. Hopefully you have not damaged the caliper trying to push them back too hard. I would suggest you install the old pads and assemble them back on the disc. Then operate the brake pedal. That should straighten them out again. You best chance to push them back evenly - just loosen the bleeder and push the whole caliper against the disk one side then the other. It can slide sideways so there should be no problem. Failing that - complete service job.
Good luck...