The web site is gone. The comments section for WoWinterface on it is closed for discussion. Is the addon dead or is the LUI site just down temporarily? I thought I remember it having some bandwidth problems and became ad-supported before. So is it officially gone now? I wanted to get to its forums to check out some technical stuff but now all I get is a web space for sale site.
I just checked the site, and it's still there. Maybe you have a old web address. I am however a member to the site. Maybe you have to set up a log in account. I don't know, but here is the web address I have. Try it and see if you get in.