I have a withdrawal error at stex.com (stocks.exchange) and no response from their support staff.
My coins have dissapeared, what can I do now? There is no mistake on my part, the receiving adress is right.
May I ask how long ago it is that you contacted support? In some cases it's "normal" that it takes a couple of days before they respond to your tickets, so have a little more patience.
Also, has there been any suspicious activity (think about trades or withdrawals you didn't execute) on your account? It may help to provide more details that may steer us into a certain direction.
I would just keep sending mails until they reply.
That may seem like a good option, but it really isn't. The more emails you send to an exchange or service, the more likely it is that they will mark you as a spammer. In other words, you only harm yourself by doing that.