And found the reason was the driver for my display, Nvidia is not detected correctly so that I installed the Nvidia driver again in my Ubuntu machine.
The following are the steps I followed to install the Nvidia driver in my machine.
sudo add-apt-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update
sudo apt-get install nvidia-current
After installing the driver, I restarted the machine and changed the resolution (Applications -> System Tools -> System Settings -> Displays) to 16:9, and solved the resolution problem :-)