Kirby G4 goes in reverse when put in drive, Pease HELP?

My husband put a fan on a Kirby G4 and had one screw left over, now it goes in reverse in drive. Could this one screw cause that? If not please help me figure out what it is. It will pick up in neutral but not as well as in drive, and it is a lot harder to push in neutral. I really need my vacuum back in working order.

Thanks in advance for your replies.