They said on their website a a few weeks ago that they would be down f or w hile. Dunno why - but hopefully it'll be back soon.
In the meantime try
http://www.cdcovercentral.com/ - it doesn't hold the covers themselves - just links to other sites so expect it to link to cdcover.cc a lot (ie. a lot of them will be broken links untill cdcovers.cc comes back)