Library Question - What can I say to defend myself?

I returned a model kit to my university library today. The kit sat on my desk, unopened, for the time I had it because I ended up not needing to use it. Tonight I got an email from the library saying that several parts were missing and that the kit is signed out to me until I can find them.
The trouble is, I know that I never even opened the kit. I absolutely do not have the “missing parts”, and therefore those parts must have been missing from the kit when I signed it out.

I am so upset. What can I say tomorrow to defend myself? Do I have any ability to defend myself at all? I’m terrified that they will make me pay to replace the kit. What do I say? :(
Any guidance would be extremely appreciated.