How do I fix an up and over door cable?

My up n over door has a standard fitting with counter weight spring above the opening, and wire cables on either side. The cables are wound over a roller at the top corners. The cable on one side has slipped off the roller. There is not enough slack to simply slide it over back into place. I have tried to lever it over back in place, no luck. the wheel is stuck in place as the cotter pin seems rusted in. The other end where the cable attaches to the door and the pivot wheel for the door on its runners is, is also stuck in place. Any smart fix.