Drywall or plaster

Okay I have this problem with my heat registers (wall). Last time the ducts were cleaned the fella took off the vents and we could not get them back on )nothing for the screws to screw into as it crumbled behind the vent. My question is, what should I do to build it back up? Should I plaster in there? Or put some drywall (not sure how to do that) or is there any other suggestion? Just looking to have something to screw into behind the vent. I'm a 'somewhat' handygal so looking for best options.

thanks