Our music collection lives on a network-shared drive. One of my ongoing annoyances with iTunes is, if it can’t find that drive because the network is down, it marks all of the music with an exclamation point icon. That icon keeps iTunes from synching the songs with your iPod or iPhone. Once the drive is again accessible, you can remove the icon by loading each track’s information individually, but there’s no good way within iTunes to tell the program, “Hey, idiot, my songs are back. Re-scan my library and you’ll see.”
There’s a workaround, though. When you start up iTunes, hold down option (Mac) or shift (Windows). iTunes will ask you what iTunes library it should load. Select your original iTunes library. iTunes will re-load it and, as a by-product, strip the exclamation point from all the files it can now find.