I have logged off from the server. (or logged out)
I am disconnected from the server.
If it's formal and technical, then it might be better to say "I am not actively using the server" for a stateless protocol (like the web), or "I have stopped using the server and my session has timed-out/expired" (like the web with cookie-based sessions). If you don't know the technical distinction of those from logging out, then it probably isn't a context where they matter, and you should just use "logged off from".