The site has been down for several hours as, apparently, its domain expired
KickassTorrents has been down for a day or so now, leaving pirates scattering to any of the other BitTorrent sites around.But it's back now, it seems that the domain name problems have been fixed and the site has been working for a few minutes now, it was showing the nginx (its webserver) welcome page only a short while ago.
Both the regular kat.ph domain and the old KickassTorrents.com, which redirects to kat.ph, are now working so anyone who doesn't want to pay for stuff can do so once again, not that it's a good idea.
The site went down yesterday, apparently as its default domain, kat.ph, wasn't renewed and finally expired. That's no longer the case, KickassTorrents has gotten ahold of the domain again and has relaunched the site.
UPDATE: KickassTorrents and several other BitTorrent sites are now blocked in the UK, if you're having problems you may find a solution here.