What does one do if he/she lost one of the 10 pieces of Bread and can't find it?
(Don't worry, just curious)
Strictly speaking, if one has 10 pieces of chametz and finds only 9 of them, one must look for the missing piece.
(If you are sure that you found 9 out of the original 10, you need to look for the missing piece. But if what you found might not be what you set out, you need to look for all 10.)
These laws are detailed in Orach Chayyim 439. For example, see the Aruch Hashulchan on the topic; see halachot 10-12 where he quotes the Tur and the Rambam.
If one searches (really looks, not a symbolic parade around the house) for the piece of chametz, and doesn't find it, and has nullified the chametz with bittul (with the formula in the siddur, for example), then that person has observed the halachot correctly and does not violate bal yera'eh uval yimmatze. Even if the chametz turns up on Pesach itself, there is no violation, as long as it's destroyed as soon as reasonably (and halachically) possible.