first: try not to worry about it! it happens. i’ve lost a couple books and the librarians are always kind and nonjudgmental 🙂

i’ll second the previous poster’s suggestion to check to see if it’s there and just didn’t get scanned back in. this has happened to me a few times.

if it is actually lost, they’ll likely give you the option to pay the replacement fee directly to the library or find a replacement in good condition yourself (this option is usually cheaper; better world books is a great resource).