Hi Dina,
Please read the instructions at the top of http://moodle.org/mod/forum/view.php?id=596 before posting.
To even begin to make suggestions as to the cause of this and to help you look into it, we really need to know more information. Have you looked at your web server's logs? These will almost certainly tell you what is causing your 500 message, and your 405.
Andrew
Hardware and performance
Loading Moodle error 500 and 405
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