I have used Mercatox in the past and never had any sort of problems (except for some small functionality issues) with the site but these problems look to be surfacing very recently. Might as well want to reach out to their support team via chat to ask for clarification about your issue.
Ive lost 50BTc and more and now this my last moneys FFS this is shit
Did you really deposit 50 BTC on an exchange with comparatively low volume in trades when there were at least over 10 better exchanges to choose from? Either ways, sorry about your loss.