its probably frozen cuz the piston is all rusted and eventually built up enough friction on the seals that it wouldn't move... my rear one was frozen... and then I took the rubber boot off and sprayed it with PB blaster and did my best to dissolve the rust. I cleaned it for the most part but its a long term temporary fix. last time I did it was 2 years ago... and then it froze again...

just replace it if its the front one... this was my rear disc and the cheapest I can find one is $137