an unknown error 404 not found

404 Not Found
This error message comes when client requests for some uri but there is no match against wanted request uri . And there is no more message comes about credential.
It may be permanent or temporary and second thing server if knows something about the request, it does not show that previous resource has been removed or forwading address.And server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.