iPad Library Within iTunes Displays Full Library But Doesn't Play & Doesn't Appear on iPad

I connect my ipad to itunes via Windows and my iPad music library lists the original 40gbs worth of music but my actual ipad music lists remains empty.

When you click on a song to play, the exclamation mark appears next to the song and no pop up window appears to say 'file location not found'.

How do i get it to show on my iPad again?