Windows and iTunes 10.
Had this problem "Error importing file .... songname .... iTunes cannot locate the required file" when trying to import a CD today.
My library is on an external USB hard drive. iTunes from time to time not only loses its own library file, but also confuses windows into thinking that the library files do not exist.
So the fix where you re-tell iTunes where to find its own media file (using the preferences menu hidden behind some opaque icon for reasons that pass rational understanding) fails, and when you try to find the file using windows explorer, windows possibly confused by apple "helper" files, or possibly lost in a delusion of its own, also thinks there are no files in the music directory. This despite usage stats on the disk still clearly showing that a lot of space is being used up by files that "aren't there".
The fix for this windows user is close everything down. Reboot, restart iTunes and presto it as if iTunes's / Window's temporary amnesia never happened.
Last time I "lost" the library was when I did the "upgrade" to iTunes 10. I wrongly put this down to a pathetic upgrade utility. I now realise that iTunes/Windows just loses this key element of the plot from time to time, screwing up windows when it does so.
Astonished how flaky this widely used and let's face it hardly brand new software combination is.
Now I don't know if this spectacularly unhelpful performance is Apple's fault or Microsoft's or both, so I'm not going to bother apportioning my contempt. Shame on both your houses for needless worry that I'd have to re-rate and re-sort everything from a virgin set of file back ups yet again.