Those are the rollers that the bed uses to align to the rail. You should be able to slide the roller up into the channel and reattach it to the bed using the bolt that should be on the floor or in the bed. Yes, you can let the bed down without it to fix it, but I would try to resolve it before you use it with folks in it. If someone is sleeping up there, all movement will be transferred to the other rail and you could damage that rail.
Look on the other side where the rail attaches and you'll see how its attached with the bolt. It will be tedius to reattach as the area is tight and an open end wrench is about all that will fit. Bear in mind, this bolt and roller keep the bed from swinging freely, so don't torque the rollers real tight, you could pull the rails out of the wall.
Michael & Angela Foret
North Central Florida
2019 Entegra Vision 31V