Q: songs on iPod have disappeared from iTunes library

Some songs that are on my iPod have an exclamation point next to them in my iTunes library. When I try to open the files, a pop-up box appears saying "This song could not be used because the original file could not be found."  I have tried to locate the song files, but they are nowhere on my computer. I have followed all the steps listed here

http://support.apple.com/kb/TS1408?viewlocale=en_US&locale=en_US

but with no luck.

How can I get them transfer these files from my iPod back to my computer? Does anyone have any idea why this happened? I think these files used to be in the library but they no longer are. Why do they disappear?

Thanks!

PJSMAC