When was the last time you changed out the fluid?
If never, I would assume you have some pretty caked up hydraulics/brakes (on the inside). In my limited experience, if you tear into the calipers and master, you will find this white crystalized crap. Generally around any of the seals, where the fluid has a bit of a chance to build up and collect moisture. The calipers would be my first check, and then the master.
Sometimes they won't work, and some times they lock up, depends on whether or not it sits or gets ridden.
Pull it apart, you can check the pads really well this way. Clean everything really well, and put it back together and I bet it'll work just like new, assuming you have any pads left.
I have been lucky in my brake work, as generally a clean up has put everything right in all cases, without new seal kits and o-rings and such. Be prepared to run into broken bits and dried out rubber. But I would bet everything will be fine with a clean up. :yes