Did you install any 3rd party modules? This is 98% of the problem... (and is random!) For some messed up reason which we or the community has yet to track down, when adding a new module (zppy, repo browser or other) the index page module list disappears
Although, this post should help you out: http://forums.sentora.org/showthread.php?tid=426
Although, this post should help you out: http://forums.sentora.org/showthread.php?tid=426