[P.P.]

here are some tips:
have you downloaded all the patches? if not do that otherwise your version isn't the same as on the server.
The server could be passworded, if so, push ~, that brings the console down, then type password ......, and press enter
or what you can do when you got back to the mainscreen, press ~, then type reconnect and press enter.
That should help you i think.
good luck