I thought cex.io was a brilliant site until...
On 5-Jan-2014 11:AM someone (IP 5.63.144.220) hacked my yahoo email and then hacked my cex.io account. The hacker sold the GHS I had and withdrew the funds. I had deposited 10 btc into my cex account. My cex account is now frozen by cex and they are investigating – whatever that means.
Did you have 2-FA enabled on your account? It's a valid question...
And other than my grandmother and an old lady at my old work, I didn't know yahoo mail was still a "thing". I thought them having malware and the service not working well this past December would be enough to sway some users...
Either way, sorry for your loss. Good luck retrieving it with the Cex crew!