Your server disappears from list when:

1) You have not validate the mail sent after registration
2) Your server was timeout during more than 20mn.
3) The 17Buddies's button with your server code has not been displayed on your webpage during last 7 days.
4) You have not visited 17's Buddies's website during last 30 days
5) You have maps (zip) or screens (jpg) to upload since more than 15 days.

To make your server visible again, fix one (or more) of the problem above and see FAQ:

Make my server visible after hidden state