Serious iTunes issue! What happened to my music?

So my boyfriend turned his computer on today and the majority of his music was missing. Just gone. Some of it was still there, and all the artist/album info was still listed in iTunes, but the actual music was no where to be found. His iTunes Music folder was just a portion of the size it should have been, and entire CDs just weren’t there any longer.

Does anyone know what might have happened to his music? He hasn’t touched his computer in a few weeks, so it’s not like he accidentally deleted it while doing something else. It’s just…gone. Could iTunes have moved it somewhere else? Any ideas? I’d appreciate any help or similar stories! We haven’t been able to find anything helpful by googling.

Also, he has Windows Vista, and iPod (an older one), and an iPhone in case that makes any difference.