Perhaps your iTunes library has been set to use a device that you don't have connected. Go to iTunes --> Preferences --> Advanced and ensure that the iTunes music folder location is on your own hard drive and not on a removable drive.
What system do you have and what OS? I had the same issue when I upgraded my father's Power Mac G5 tower from 10.3.3 to 10.3.9. I found out thanks to Software Update that I needed to apply a firmware update and once it was applied, I was able to mount the disk images.