Forum Moderators: open
You can inquire at the Resource Zone. A link is available in the Forum Charter [webmasterworld.com]. This is the best place to get an answer. All we can do is guess. :)
The content of the site may have altered (for example affiliate links added after inclusion), it may have been 404 for a while, or simply another editor may have deleted it as not having enough original content. The reasons for deletion are many and various.
However if deleted, a permenant record is left of who deleted it, and why. This is what you need to find. Follow the above suggestion to have a go at getting to the reason.