Do you know where on your computer your song files are?
Your songs might still be there, but iTunes just can't find them because something might have changed.
If you know where they are, then just do a quick check to see if the song files are there.
If you're not sure where they are, you can try looking in your music folder. Press Start, and click Music.
Do you see your music in that folder?
If you don't, then you can use this support article to find your iTunes Media folder. Your songs might be in that folder.
If that still doesn't help, then the steps in this article might help you track down your music.