Installed dual os- windows and ubuntu, but not able to see ubuntu?.. then try this

If you have a Ubuntu Live CD , then start Live session . In The Live session , open your terminal and type this sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update   Give a Enter shot and try this sudo apt-get install -y boot-repair && boot-repair   then it will open boot-repair as shown in image there select Recommended Repair Option . It will do the job of restoring your GRUB and then give a restart . remove your CD . There you go with your GRUB . Source :https://help.ubuntu.com/community/Boot-Repair