Suddenly of the pages has the access denied error causing from SPSecurity.RunWithElevatedPrivileges delegate method. Just to highlight that this delegate method that is to run with elevated rights of a Application Pool identity (NOT primary administrator). Many new SharePoint developer still having this misleading information and I seen many like to abuse and massively use this SPSecurity.RunWithElevatedPrivileges delegate method.
Back to my solution, I checked the Application Pool identity the user has been changed and does not have any permission in my site collection. Well, I working in a shared environment, sometimes people just change here and there without notification. Granting that account full control in my site solved the issue.