A whole bunch of my purchases music is gone from all my folders/library/devices. I can see my purchase history under account info, but can't figure out how to GET the music back on my devices!

A whole bunch of my previously purchased ITUNES music is missing from my playlist...I can see it in my purchase history but nowhere else. I am talking 100's of songs. So sometime along the way when I was syncing or transferring music, something went wrong...how do I get it back?? It won't let me download again...