I installed Ubuntu in a Windows 7 PC in its
G: partition. But after installation and restarting the PC, there is no option for boot to Ubuntu. I am not a computer expert. Please help me. During installation how do commands work.
Install it on your main drive, usually /dev/sda.
then use the command
It should automatically see your Windows partition, and you'll be rocking a dualboot.
Another option is to restore the Windows Bootloader after installing Ubuntu, and then proceeding to use EasyBCD to add a boot loader entry for Ubuntu.
I triple-boot OS X, Windows, and Ubuntu with the Chameleon Bootloader.
It automatically recognizes all my operating systems (all on separate drives within the same computer)! Plus it has a GUI. I love it!
If you installed Ubuntu using a USB, try booting the computer with the USB connected.
EDIT : Extra info as requested
Hold "ctrl & Alt" then press "T" and that will open a terminal.
As suggested above try booting with the installation USB connected and let me know what happens.
Boot into Windows and use EasyBCD to fix/adjust the boot setup.
Here is the website for EasyBCD.
Also, take a look at this answer on AskUbuntu. It describes how to setup EasyBCD for boot problems.
Edit: It could be a graphics driver problem, give this answer a read as well.