Help i have no music

Hello,

I needed to change hard drive so exported my itunes library to an external hard drive and then one the ne whard drive was in i imported the library. Now i can see all the music but none of it will play or download to my ipod - looking at around 8mb of content. The system is blocking it saying it cannot authenticate the origin of the music. It is all legit - no illegal downloads.....any advise on this one???