I turned on directory browsing (in IIS) for my sharepoint site, then turned it off again and now every time I try to access the site through a web browser I get 404 error, page cannot be found.
I know it's something to do with authentication but don't know where to start looking/fixing ?
More of a work around this:
Rather than try to figure out what permissions to set/reset, just delete the web application, specifying Yes to 'Delete IIS Web' sites and No to 'Delete content databases'.
This will keep the content database (with all your content you can't currently access).
Then create a new web application with the same name and host header and enter the existing content database name.
This should then recreate the IIS web app with correct permissions and connect to the existing content database.
Can you give a bit more background? Do you get 404's for all pages? Can you hit layouts pages? E.G:
http://YOURSITE/_layouts/settings.aspx
404's in SharePoint shouldn't be taken at face value. Check your ULS logs in:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\LOGS
We ran into a smiliar issue yesterday