Stake asked the OP to provide his source of funds statement. They have suspended his account and requested for those documents. OP will be able to access his account again if he can successfully go through the verification process.
If it is so, they're stealing from OP. They have no right to keep his money regardless of whether he can prove it was earned by him through mining or whatever else.
Imagine that he was given this money for free and cannot prove that it's legit, it's still his money. No company has the right to act like a judge and jury and steal that.
I don't know how Stake will approach this but there's only 2 legal ways. Either they accept the money, or return it back to OP.