If your system does not boot for any reason, it may be useful to load it into recovery mode. This mode only loads some basic services and takes you in command line mode. Then you are logged in as root (superuser) and can restore the system using command line tools. When booting in recovery mode does not work, you can still use the installation CD (if it is a live CD)

Download in recovery mode

1. Turn on the computer.
2. Wait for the BIOS to complete the download, or almost complete. (During this time, you will probably see the logo of your computer manufacturer.)
3. Quickly press and hold the Shift key, which will open the GNU GRUB menu. (If you see the Ubuntu logo, you missed the moment when you can enter the GRUB menu.)

4. Select the line that starts with " Advanced Options ." "Advanced options"

5. Select a line ending in "(recovery mode)" " (recovery mode) ", possibly repeating the line, something like:

     Linux, Ubuntu GNU / Linux, with Linux 3.8.0-26-generic (recovery mode) 

6. Press Return and your machine will start the download process.

7. After a few seconds, your workstation should display a menu with many options. One option (you may need to scroll down to the bottom of the list) would be Drop to root shell prompt ". Press Return on it

8. The root partition is mounted read only. To mount it for reading / writing, enter the command

     mount -o remount, rw / 

9. If you have / home, / boot, / tmp, or any other mount point on a separate partition, you can install them with the command

     mount --all 

   (This should be done in the next step 8, so / etc / mtab is writable.) In addition, you can try to select "Enable networking" in step 7, although this was determined to cause the system to hang (see https://bugs.launchpad.net/ubuntu/+source/friendly-recovery/+bug/981792).

Note 1: When rebooting, there is no need to hold down the Shift key, in step 3. The GNU GRUB menu will still be displayed automatically on every boot until the normal launch of Ubuntu is complete. To configure GRUB, a menu is always displayed, even after a normal launch, see https://help.ubuntu.com/community/Grub2/Setup.