I already used Manual Upgrade. Changed theme to Default TwentyFifteen, but still Notice displayed.
I found the issue with one of the plugins when active, I am dealing with their support.
But strange thing is the same plugin works fine with my other website.
Just download the theme again, extract the files under /wp-content/themes, this will recover your website and Dashboard.
In that case, you should get help from some professional Server Management team. So that they can recover your data.
Thanks that worked.
Please do let me know when will there be the update available.
Also when I update the plugin, do I need to remove the code which you provided?