The page you requested is NOT AVAILABLE
This looks like a server error. I suggest checking permissions to start with. Then provide more information - server specs/os, Moodle version, any other error with debugging turned on, what happened 3-4 days ago, any new plugins etc?
I am using moodle's 2.6 version, and I don't think any changes are applied to it. The moodle settings are by default. 3-4 days before it was working fine, and then suddenly this happened. No updates/ plugins are installed even before the error and after the error. Is their any way out? what steps can i take to solve this problem?