Most of the times I run into this was because the servers the site was on were being repaired or run into technical problems. Then a few hours later - the site was back on cause they fixed it.
Sort of like going to the bookstore and discovering it was closed for renovations.
Sometime this means that the site had been either removed or moved to a new server or is running under a different name.
I guess the equivalent in the off line world would be going over to a book store you haven't visited for a while was closed and htey are opening a shoes store instead.