Garage upNover door ?

I'm also placing in General DIY
Garage up and over door pivoted centrally by roller and pin in a riveted on bracket... 1 each side.
Each pin also retains the cable which I assume is assitant to opening as its unde tenson when door closed. One of the rollers has come off as pulling down on the door hard it went cocked to one side and the roller flew off the pin. How do I get the roller back on the pin? Many thanks in anticipation.