missing itunes music collection

What happened to my extensive itunes music collection? I simply cannot find it anywhere! It used to be in the cloud, and it's not there either! Help me somebody, anybody! All ideas are welcome! I've just started digging into it again after giving up finding them, some of those files are irreplaceable after I thought that by committing them to the cloud that they would be safe and always accessible until they were not! There's gotta be a way...