If your maximum downloads are set at zero this will happen. Tools > options > downloads. Click on 'apply' after making any changes.

This prob can also happen if you're trying to download lots of files at once. Try limiting to 2 or 3 simultaneous downloads. I think that you can get stuck on 'queued' if you use the resume too much, as well. Use the search (top right, between faq and home) to find out more