Dig up your old deposit/withdrawal history if you can, and include the txids of your deposits/withdrawals.

Without proof, I doubt that yobit support will bat an eye at this case, though they're likely not going to respond in any helpful way even if you do have proof. Just saying that the amount vanished from your account is simply not enough to hold them accountable.

Also, make your case clearer - did you have your funds in investbox, or did you have it in your BTC balance when it disappeared?

Just so that you know in the future though, this kind of scam doesn't come as a surprise to anyone. The whole InvestBox concept is extremely sketchy, since it's basically offering ponzi-like returns. And yobit is renowned for their support, in a negative way.