You haven't lost them, you can download them all again without having to pay for them again. It might be a pain in the butt to have to do it, but you will get them back. The forts thing that I would do is to reset the iPad and see if the songs come back.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.
If they don't come back, go to the purchase tab of the iTunes Store and tap on the Music tab at the top. Them tap on Not on this iPad. You will be able to download all of the songs by tapping on the close icon next to the song.