Does it stil have the cloud symbol or not ?
If it has the cloud symbol next to it e.g.
If it has then that implies that it is only in the cloud and not downloaded into your library. If you click on it then it should download onto your computer.
(You can turn off whether items that are only in the cloud show on your iTunes library via Edit > Preferences, iTunes > Preferences if on a Mac - select the Store tab on that and there should be a 'show iTunes in the cloud purchases' tickbox.)
You can also redownload content via the Purchased link under Quicklinks on the right-hand side of the iTunes store homepage. If an item is shown there but doesn't have the cloud symbol then is it still listed in your iTunes library (i.e. where you would click on it to play it) ? If it is then iTunes will be seeing that and assuming that there is therefore no need to be able to redownload it - deleting it from your library should then give the cloud symbol against it in Purchased.
If it doesn't show in the Purchased link at all then is it still in your country's store ? If not then the record label has probably removed it, so you won't be able to redownload it. If it is still in the store but not in Purchased then check to see if it's hidden : http://support.apple.com/kb/HT4919