-Is it failing to refresh the downloads list (e.g. package database update), or failing to download a program itself?
-Have you tried it on a different network? (presuming you're using wifi, otherwise try wifi)
-Can you load http://repository.maemo.org/ manually in the browser?

The maemo.org servers did have some downtime a while back, but I think they got that all fixed.

Also, if you post the actual error message, verbatim, it usually goes a long way to getting quick help.