The piece under key hinge –– that looks like a nipple –– came loose. What to do?
The piece is the "dome" - it sits under the key cap (see here). Underside of it has a conductive surface, so pushing it down closes the contact to trigger a key press. You may be able to fix this with some very small bits of double-sided adhesive tape; not sure I'd superglue it, because failure to fix could mean you're looking at a total keyboard replacement, depending on where the glue gets.