I loaded new music from the itunes store, and it loaded them into itunes, but apparently deleted (or hid?) the existing library. I have the feeling that the music is 'there' but cannot see it.

This is going to sound really stupid, but are you sure you're not just in the Purchased Music playlist? (See now, isn't that stupid and belittling? So sorry)

One thing you could try is look for the actual files of your music. If you have a Mac, go to your Mac HD, into your music folder and look for the iTunes folder (It's probably something like this for Windows). Open it and open the iTunes music folder. Are there folders there that have titles of some of the artists that you lost? If so, the music is somewhere in iTunes! (Hopefully) Open one of the artists that you "lost" and if there is music in there, then drag it back into iTunes. I know it's a pain, but at least you don't have to reload your music.

Anyway, good luck, and I hope you find your music :D