How to Repair GRUB2 in Ubuntu (Graphical method):

Ubuntu and many other Linux distributions use the GRUB2 as the default boot loader. Suppose, if you convert your Citrix Xenserver Ubuntu VM to VMWare ESXi VM (not always), or if you install Windows after installing Ubuntu, or if you overwrite your MBR, then you won’t be able to boot into Ubuntu.

In any of the above cases, you can use Boot Repair tool (which is a graphical tool) to easily restore GRUB2 from an Ubuntu live CD or USB drive.

Note: You need an internet connection in the server from live CD to achieve this.

Boot-Repair is free software, licensed under GNU-GPL. 

There are two ways to get Boot-Repair:

1st option: Get a CD including Boot-Repair

The easiest way to use Boot-Repair is to burn one of the following disks and boot on it.

1.      Boot-Repair-Disk is a CD starting Boot-Repair automatically.

2.      Boot-Repair is also included in Linux-Secure-Remix.

Remark: you can also install the ISO on a live-USB (eg via UnetBootin or LiliUSB or Universal USB Installer).

2nd option: install Boot-Repair in Ubuntu

- boot your computer on a Ubuntu live-CD or live-USB.

- choose "Try Ubuntu"

- connect to the Internet

- open a new Terminal, then type:

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

- Press ENTER.

- Then type:

sudo apt-get install -y boot-repair

- Press ENTER

Follow the below steps to repair GRUB2:

Step 1: If you have the media you installed Ubuntu from, insert it into your computer and restart.

If you don’t have, download a Ubuntu live CD and burn it to a disc or install it on a USB flash drive. You can also download a dedicated Boot Repair live CD.

Step 2: After booting into the live Ubuntu environment, open a terminal and run the following commands to install or open Boot Repair:

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

Step 3: The Boot Repair window will appear once you run the boot-repair command. Click the “Recommended repair” button to repair GRUB2 with a single click.

Step 4: Restart your computer after using the Boot Repair tool. Ubuntu should boot up normally.

Thanks to Boot-Repair and RecoveringUbuntuAfterInstallingWindows