Look for a folder called "Downloads" in your iTunes Music folder. If there is such a folder, look for files that would be the partial download and delete those, so that the download starts over from the beginning. Then try again using the "Check for Purchases" command under the iTunes Store menu.
If that doesn't work, contact the iTunes Store customer support department through the form at the bottom of
their web page and explain the problem to them.
Hope this helps.