Today, Mt. Gox issued a press release on what happened.
Apparently, the exchange has been subjected to a massive "distributed denial of service" attack.
"Since yesterday, we are continuing to experience a DDoS attack like we have never seen," the company said on its website. "While we are being protected by companies like Prolexic, the sheer volume of this DDoS left us scrambling to fine-tune the system every few hours to make sure that things don’t go beyond a few 502 error pages and trading lag."