Well, it was the biggest exchange in the world at the time by far and we'll newer know what exactly happened, was it a hack or inside job, but it suddenly closed...
"Mt. Gox was a bitcoin exchange based in Shibuya, Tokyo, Japan. Launched in July 2010, by 2013 and into 2014 it was handling over 70% of all bitcoin transactions worldwide, as the largest bitcoin intermediary and the world's leading bitcoin exchange."
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"By May 2016, creditors of Mt. Gox had claimed they lost $2.4 trillion when Mt. Gox went bankrupt, which they asked be paid to them. The Japanese trustee overseeing the bankruptcy said that only $91 million in assets had been tracked down to distribute to claimants, despite Mt. Gox having asserted in the weeks before it went bankrupt that it had more than $500 million in assets. The trustee's interim legal and accounting costs through that date, to be paid ultimately by creditors, were $5.5 million."
https://en.wikipedia.org/wiki/Mt._Gox