Eric, iTunes did not make your music files invisible, you're just not looking in the right places.
Start up iTunes, go to the Edit menu, select Preferences, select the Advanced tab, and you should see something like this:
Here you will be able to see exactly where iTunes is keeping all the music.
Alternatively, you can use Windows search feature to find a folder called "iTunes Music". Open the fold up, and all your music should be in there arranged in different sub-folders.