On our 2016 Grand Reserve, it had a cable system BAL slide mechanism. I never had to use the manual mode, but remember that I was provided a flexible cable that attached to a drill and the other end to the mechanism on or near the motor above the slide in the interior of the coach. There was a wood panel on top of the slide that could be removed to give access to the motor area.
Is the motor making a noise? I had a screw come out of our motor and that could potentially cause the chain to come off the gears if you have the same type.
Like Johnboytoo said, check your manual if you have one, or check Youtube or the company that makes the slide mechanism. Crossroads or a RV dealer may help, or a mobile tech. Good Luck!
Steve & Sue
2017 Grand Design 297RSTS Travel Trailer
2017 Ford F-350 Lariat Crew Cab diesel 6.7 SW LB 11,500 lb GVWR package
Former: Crossroads ST26RB