When you see this on the log in screen it occured for one of two reasons.
The first is that the game just had a system update. Give it a few minutes and try again.
The second reason would occur if something happened to crash the game. This usually occurs right after an update from Jagex.
An example would be when they released the update on March 17, 2009. One of the minor changes they did not list caused their bandwith to be overused, thus causing massive server crashes. When this happens, do not try and log in. Furthermore, if you are not sure that this is the reason, try searching the Runescape forums under "Recent Updates" to see if there are any posts from Jagex Moderators concerning the issue. If you discover that this is the reason, either wait to log in for some time, or keep watching forums for an update that it is fixed.
Many crashes have occured lately in the game due to technical updates that had a slight issue, but caused major problems in game. For furher info on this subject, be sure to look in the Runescape forums, or the Help options on their webpage.