How to show GRUB after install Ubuntu over Windows 8?

Install Boot-Repair on ubuntu by following steps

Open the terminal and run the following commands

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair

After completing the installation you can launch it from System->Administration->Boot-Repair menu if you use Gnome, or search "boot-repair" in the dash if you use Unity. Then follow the following screenshots:

Method 1

• Click on the advanced options

• Tick the options shown below

• Change the tab to Grub Location Tab and Tick The options Shown in the figure

Press Apply and Reboot the system

Method 2

• Select the recommended Boot repair options as shown in the first screenshot