If you are playing Warcraft III: The Frozen Throne and you receive a blank error box with a "!" when trying to join or create a ladder game, this indicates your CD-Key has been temporarily or permanently banned from playing ladder games on Battle.net. The missing error message on The Frozen Throne will be addressed in the near future (Reign of Chaos displays the correct error message).
Warcraft III CD-Keys are regularly banned and accounts are closed due to hacking on Battle.net. In the case of the CD-Keys that were banned, at least one account tied to each CD-Key was identified as having used such a program while playing on Battle.net. First offenders are banned from ladder play for one month and repeat offenders are banned from ladder play permanently. Users who are unsure if their CD-Key was temporarily or permanently banned will have to wait one month from the time the CD-Key was banned to determine the status.