One of two things transpired - perhaps both.

1. The 20 minute timer built into the program expired, and brought you out of the program. If enough time has passed, the login timeout message might be replaced by your screen saver, and the browser would try to connect back to the web source, leaving a Server Not Found Message.
   
   
2. Your logon to your ISP expired, perhaps due to inactivity, perhaps selected as a timed event. Reconnect to the Internet, log back in and continue working, there should be no ill effects from a timeout incident.