Sometimes network connections are lost without you noticing. This is especially true with Jabber, as it is quite reasonable to keep the connection open for a long time without either sending or receiving any data.
If you want to detect a lost connection earlier, you can use the keepalive functions. Type M-x jabber-keepalive-start to start it, and M-x jabber-keepalive-stop to stop it.
These functions work by asking your server for the time once in a while (by default every ten minutes), and considering the connection lost if the server doesn't answer within reasonable time (by default 20 seconds).
You can customize the interval and the timeout with the variables
jabber-keepalive-interval and jabber-keepalive-timeout,
respectively.