I play Quake 4 online and lost all my servers for 4 days.They came back on their own with the only thing i had done was deleting my cookies and temp files from internet explorer.
The Quake forum has lots of people with the same problem of servers dissapearing and reappearing sometime later of their own accord.
Hopefully yours will reappear.
Try rebooting if you leave your pc permenently on and delete all cookies and temp files.
Have you installed a software firewall lately?If so try dissabling it or turn off all firewalls just to see if it's some property within them thats changed and preventing it.