Q: Itunes downloads lost

I downloaded Coldplay music off ITunes a month a go using a voucher. Everything worked fine I recieved the music until today.

An exclamation mark is showing next to each of the songs and when you go to play them a box displays saying:

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

I have looked for the files on the computer and could not find them.

Any ideas on where they might be or how I could fix this problem?

Thx