This server split off from the part of the net you were on. Note that netsplits are all from a point of view of the user. After a netsplit rejoins people might say to you “where did you go?” — because from their perspective, *you* split off.
– irc-faq; Version 1.53; November 28, 1995.
An IRC network is made up of potentially dozens of servers communicating messages back and forth as fast as possible in order to appear like a single server. Sometimes a network will split into two networks in order to balance its communications load, right in the middle of thousands of conversations.
If you experience a network split, you might well lose communications with some channels or users that end up in the other half of the split. There is nothing you can do about a network split, so carry on chatting and wait to see if the network rejoins itself.