How can I boot to Live CD and then access Ext4 partitions.
If you don't have the Ubuntu Live CD, go find another computer, download the image and burn it on a CD/DVD.
When you've done that, or if you already have the live CD, make sure your BIOS is set to boot from CD/DVD before booting from HD.
When your pc is off, put in the Ubuntu Live CD, close the drive and startup your pc. When you are asked, choose "run live CD" and wait until you see the desktop. In the Live session, you can access any HD, USB drive, etc... you want. If you want, you can reinstall the Ubuntu OS.