They're moving servers. People have been getting different DNS results. One leads to a plesk control panel another leads to that search page... Or they could be gone for good, but it sounds like they're just moving to a better server.
I also used to use tvnzb.com. My guess is that the website will be gone forever, so I decided to create a similar website. It's http://medianzb.com and basically gives you a custom feed based on shows you select.
Instead of using user submitted nzb's, it uses automatically fetched nzb's from nzbmatrix.com. These are uploaded by users on that website and are of good quality.