I was on a server called 'unibets' earlier when i suddenly got the 'not connected' message.

I went to the server list and the server is no longer listed..
I did some player searches of players, that reported the other players were still in 'unibets'.

Why is this so? Player search says they are still there but the server no-longer listed?

That means that it had ghosted.

I think it is because of server overflow. I really don't know the reason.

Go in to #support and ask.
http://forum.toribash.com/chat_view.php?channel=support

The room might be set to private mode or is simply full. If you want to connect to an existing server by name, type /jo [Server name] ingame. Servers sometimes fall out of the list, and it's nothing to worry about. You might also have been kicked from the server, if you suddenly got "not connected".