One of my customers recently received some bad advice:
Why don't you rename that "SiteDirectory" site in MOSS to just "sites" so it will be shorter to type for your users?
Many of you may already know that the "sites" top level url is reserved by default in MOSS. But what you may not realize is that it is quite easy to change any site's url to use the reserved site names. You will not be prompted that this might be a conflict.
So, what happens? Any of the sites you formerly had under SiteDirectory will now generate an error when you try to browse to them. In addition the Sites tab that shows up in a standard collaboration portal template will fail.
If this does occur, don't despair. Follow these steps and you can get it reconfigured properly and not lose the sites.
The sites should now be back to normal.