If the content was unprotected
MP3 content, you'll need to restore the file from your own personal backups. The same is true for other stores that sell unprotected content, like Amazon MP3 and iTunes (Plus). (The providers only allow a single transfer/download, and it's up to you to keep the file safe.)
If they are protected WMA, they should be able to be restored if you are within the download limit (5?) and they are still available from Marketplace.
If the tracks are on your Zune device, you can 'reverse sync' it back to your PC, too.