[afterburner]

If you lied, and they found out, you would have been fired for that reason alone. On the other hand if you told the truth right away, it's hard to say if you would have been fired; where I've worked you wouldn't have been fired for telling the truth and doing that (having seen people fess up to much worse failures). Lying on the other hand is a serious problem since it betrays trust (if caught, of course).

That said, I've also been at a company where people in charge had admitted to not having a backup copy of something rather important. I was flabbergasted.