As others have stated, check the wheel bearing first. Failing that,
check to see if the caliper mounting pins are free. Work the caliper
back and forth to see if you can get the piston to retract a bit. If
not, it may be that the piston is stuck. Remove the caliper and use a
large c-clamp to push the piston back in a little. If it won't go, you
probably have a stuck piston. You will need to disassemble and
clean/reseal. If the piston goes back in easily but the caliper
pins/hardware are binding, you will need to clean that out.

Good Luck,
Mike
My brakes have been dragging ever since I got our Cessna 210 back from annual last month.