grub2 - Recovering Grub after installing windows 8.1

I installed Windows 8.1 after Ubuntu 12.04.

Now, grub does not shows up any more! It's goes straight to Windows after turning my PC on.

Grub gone after installing Windows 8.1

Please tell me what should I do to turn grub back ?

Any helps would be awesome.

You just need to install grub again with an Live CD.
This link could help you