Non-playable purchased songs from 2004 in iTunes library

I was probably one of the first customers at iTunes. This was the time of Napsters and when they dominated internet. mp3 songs where one song took 2 minutes to download. Then came iTunes. I bought a lot of music back then. I still have them visible in my iTunes library but they are not playable. iTunes skip them when I try to play these songs. I am talking purchases I made in June 2004 for example. I spent hundreds of dollars in purchase and they are today not playable. Some album is not even found on iTunes. how can I get help get the songs back?

[Re-Titled by Moderator]