Seized pistons the likeliest culprit, you can start by pushing the pistons out a little bit then giving a good scrub with a toothbrush and brake cleaner, then smear the pistons with red rubber grease.
Normally caused by baked on brake dust, or simple neglect.
If that doesn't work then it'll need a proper overhaul and clean up with new seals too.
If you put the bike on the centre stand does the rear wheel spin freely, or is it rubbing badly?