Tags are labels you assign to music files to make them easier to work with. Things like genera, title, artist and year... etc, are some examples of popular tags. It sounds simple, but you need to be careful. Its easy to confuse tags with the info you see in your file manager. When you are looking at tags, its usually called library view. When you are looking at music files in your file manager, its not the same thing. For example, if you are in iTunes listening to music by The Rolling Stones, the tag for the artist is The Rolling Stones. Now, if you leave iTunes and access the music files directly, the artist could be Rolling Stones instead of The Rolling Stones, like you see in iTunes. Tags and file info have nothing to do with each other. They can be different. I bring this up because it can drive you crazy if you don't understand what's going on. Someone will change or edit tags in a program like iTunes but when they look in their file manager, nothing has changed, and they think something is wrong.
In your case, I think its very likely, iTunes changed some tagging info, file info or both. Using The rolling Stones as an example again, in iTunes, you may have tagged "The Rolling Stones". If iTunes re tagged some of your music using the name Rolling Stones instead, its now a different band in iTunes. And that's how you loose your music. In this example, you would fix things by changing the tag Rolling Stones to The Rolling stones. Once the tags are labeled properly, your music should show up.
If you need good tagging software, download a free music player called Quod Libet. Since you didn't say what OS you have I chose Quod Libet because it works on Mac, PC and Linux.
