Can't play CDs uploaded to iTunes...

I have several Oasis albums. Some are downloaded from iTunes and some are uploaded from my old CD collection. I now can't play the ones uploaded from CDs, and when I try to play them this notice comes up:

The song "Be Here Now" could not be used because the original file could not be found. Would you like to locate it?

Could anyone help with a step by step guide as to how I can locate and reinstate these albums please? Otherwise it's a long search for the old CDs again.

Thanks,

Off Beat.