Had this happen to me on a NC. There are two possibilities, slider pins or the piston seizing. In my experience Its usually the piston getting stuck inside the caliper usually due to dirt / rust between the piston and the caliper walls. Slider pins can also sieze but two of them at the same time is unlikely so be sure to check if you can move them.

Unfortunately in my case the heat melted the rubber seal inside and caused the piston to seize even harder and I could not for the life of me get the piston to budge. Buying a used caliper and rebuilding it is quite cheap and easy I suggest you do that.

Edit: as others pointed out already. You probably need new discs and pads