Some simple steps to install/recover Grub with a Linux Live CD/USB after installating another operating systems or when Grub doesn't load. It has been tested with Ubuntu and Debian.

First of all boot your Linux Live CD/USB, open Terminal and enter following commands:

1. This command for root permissions:

   sudo -i
   

2. Check the drives number in Partition Manager:

   fdisk -l
   

3. Select the drive where your Linux installation resides. Change sda with your hard drive (it can be sdb, sdc, etc) and the 1 with your drive number:

   mount /dev/sda1 /mnt
   mount --bind /dev /mnt/dev/
   mount --bind /sys /mnt/sys/
   

4. This command will chroot into your installation:

   chroot /mnt
   

5. Install Grub and change sda with the hard drive where you want to install Grub:

   grub-install /dev/sda
   

6. Optionally you might want or need to rebuild the Grub menu:

   update-grub
   

Now the installation is finished. Reboot your pc and Grub is back. That's it!