If your chair back got loose, there’s an easy
way to fix that. All you need to do is to fix this screw.
For better view, I’ll remove the seat. You don’t have to do that.
Put a drop of superglue on the thread, then slightly unscrew the bolt
to spread the glue more evenly along the thread, and screw it back
very tightly. Give it half an hour to cure, replace the seat and you
may continue to use the chair. If you don’t want to remove the seat, just
apply superglue around the tip of thread and tightly screw the bolt back
in. If you ever need to unscrew that, apply some
nail polish remover to the threading and let the glue dissolve. You should
easily unscrew the bolt after that.