Probably the 'wall' was held in place by staples into the back sides of the surrounding cabinets or furring strips. Since you probably have no access from the back because of the fiberglass end cap, you'll have to pull it forward into position and perhaps screw from inside cabinets (on sides or overhead) or use construction adhesive or other glue. To pull it into position you could use a screw or a bent piece of wire or metal strip to pull it forward. You could fasten a trim piece across the top or sides to cover marks on the panel after moving it into position and glue/screw the panel to the trim.
I had a space open up behind the drawers in a night stand. I was able to screw it and then seal the gap with expanding foam insulation. I discovered it when things in the drawers were covered in soot from the engine.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI