How long did you wait? The data doesn't just magically appear right away.
If you've sat there for a couple hours with zero activity, try something else. Stop the torrent, then go into your downloads folder, copy the data and paste it (select main directory of your torrent data, Ctrl+C, Ctrl+V). If it asks, create a copy, don't merge. When it's done cloning the data, delete the original and rename the clone to the original. Alternatively, copy into a different folder, delete original, move (cut and paste) the copy back into the original location. Go back into your client, recheck data and resume torrent. If there was some HDD issue, this will likely fix it by forcing the data onto a different part of the drive (and if it does, it's probably time to get that drive checked out).