Ubuntu fails to start after I’ve installed Windows

Occasionally you may install Ubuntu and then decide to install Microsoft
Windows as a second operating system running side-by-side with Ubuntu.
While this is supported by Ubuntu, you may find that after installing Win-
dows you may no longer be able to start Ubuntu.
When you first turn on your computer, a program called a “bootloader”
must start Ubuntu or another operating system. When you installed Ubuntu,
you installed an advanced bootloader called "bootloader" that allowed you to choose
between the various operating systems on your computer, such as Ubuntu,
Windows and others. However, when you installed Windows, it replaced
GRUB with its own bootloader, thus removing the ability to choose with
operating system you’d like to use. You can easily restore GRUB—and regain
the ability to choose your operating system—by using the same CD you used to
install Ubuntu.
First, insert your Ubuntu CD into your computer and restart it, making sure
to have your computer start the operating system that is on the CD itself (see
Chapter : Installation). Next, choose your language and select Try Ubuntu.
Wait while the software loads. You will need to type some code to restore
your bootloader. On the Applications menu, click Accessories, and then click
the Terminal item. Enter the following:

$ sudo fdisk -l

Partition table entries are not in disk order
This output means that your system (Linux, on whith Ubuntu is based)
is installed on device /dev/sda, but your computer is booting to /dev/sda
(where Windows is located). We need to rectify this by telling the computer
to boot to the Linux device instead.
To do this, first create a place to manipulate your Ubuntu installation:

$ sudo mkdir /media/root

Next, link your Ubuntu installation and this new folder:

$ sudo mount /dev/sda1 /media/root

If you’ve done this correctly, then you should see the following:

$ ls /media/root

Now, you can reinstall GRUB:

$ sudo grub-install --root-directory=/media/root /dev/sda

Installation finished. No error reported.
This is the contents of the device map /boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script grub-install.
(hd0) /dev/sda

Finally, remove the Ubuntu disc from your CD-ROOM drive, reboot your
computer, and enjoy your Ubuntu system once again.
this guide may not work for all Ubuntu users due to differences in system
configuration. Still, this is the recommended method, and the most suc-
cessful method, for restoring the GRUB bootloader. If following this guide
does not restore GRUB on your computer, please consider trying some of
the other troubleshooting methods at https://help.ubuntu.com/community/
RecoveringUbuntuAfterInstallingWindows. When following the instructions,
please note that your Ubuntu installation uses Grub. This guide replicates
the method described in the first section of the referenced web page. Please
consider starting with the third section, https://help.ubuntu.com/community/
RecoveringUbuntuAfterInstallingWindows.