Sometimes if the server is full it wont let you logon....
But this is Kain, its never full. :D So maybe the server was having a hicup or something. I'd suggest waiting for a while and trying again. If it isn't up by the next day than its either a huge bug with the server or you have something wrong with you computer or client. After a while you can send a message to Lineage 2 Support and they should say something about it if it is a real problem.