It could potentially be 1 of 2 things..
1) Someone has linked to you and that has affected your PR, in which case you will probably find that the domains PR changes back after the next update in around 18 days time or;
2) The domain has been blacklisted by Google for some reason.
Can you still find search results for the domain if you search..
link:
http://www.domain.com in Google?
If so then i would lean towards option #1 above.
Regards,
Lee