Once I dismissed that error message, I was then presented with another one, reading "There was an error loading the theme, and the default theme could not be loaded. Attempting to start the standard greeter"
Finally, after dismissing after that error message, I was taken to the plain ol' GDM standard greeter (without any theme), where I could log in.
Once logged in, there was several other error message such as "The icon redhat-email.png could not be open" & "The icon redhat-browser.png could not be open"
So after much googling manage to find its a bug in the system. So to overcome this, the suggestion was to install an rpm file.
The rpm file is located here.
The download process is as per normal. So is the installation. Once the .rpm file is installed, you can see the email & web-browser icon is displayed. I even reboot to check whether the login screen is restored and yes it is.
So thanks to the CentOS people, i solved my problem.
More details on the bug is located here.
Hope this helped those of you who is having the similar problem. Thanks n together lets make open source better.