In recent years, KickassTorrents has become one of the biggest and most popular BitTorrent sites out there, especially as some of the old popular ones were shut down or disappeared. And so far, it managed to stay out of legal trouble.
So when the site went down earlier today, it's easy to understand why people thought the worse, as is usually the case with BitTorrent sites.
But the problems are technical ones it seems, the site is actually working, but the kat.ph domain it's been using more recently expired a couple of weeks ago and has now stopped working altogether.
The site is still working via the old domain KickassTorrents.com, ironically the one it switched from for fear of getting taken over by the US authorities. The site told TorrentFreak that things are being worked out and the problems will be fixed eventually.
UPDATE: The site is back up, both domains are working and everything seems to be in order. More details here.
UPDATE 21 March 2013: The site, along with H33T and Fenopy, is now blocked in the UK, so if you're experiencing problems and you're in the UK, this is what's going on. There are ways of bypassing the block though.
UPDATE June 13 2013: Kat.ph is gone, probably permanently. KickassTorrents seems to have lost the domain, but thankfully it found a new home. You can find out more here.