I can't CHMOD file index.php

[HB10a8]

Sorry, my english is verry bad, I hope you will understand.
My site encountered an error:
Forbidden

You don't have permission to access /diendan/index.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.11 (Unix) mod_ssl/2.2.11 OpenSSL/0.9.8i DAV/2 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at htk.vn Port 80 403 Forbidden
But when I CHMOD index.php to 777 by cuteftp (now, that is 0), I encountered an error:

http://vinh1.org/images/dzbu7qwfnxmnarfrwniw.jpg

Please help me

[Dismounted]

HB10a8,

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