What version of iTunes did you download. I haven't updated recently - the version I have installed is 7.5.0.20 - but I've been having this problem. But it seems to only lose part of the library. Today, I had to reload Vivaldi: 4 seasons, Dire Straits: Making Movies, & Silly Wizard:Wild and Beautiful. Everything else played fine.
Sii, thanks for the tip - that'll be a lot quicker than right-clicking on every song and searching for it.