Sometimes after a kernel upgrade, I don’t know why, the grub is not updated properly and then it is not possible to boot the system. Here it comes the way to solve this problem quite easily.
Use a LiveCD or a Rescue Mode to have access to the hard drive that holds the OS you are not able to boot.
Once logged in, chroot your former system.
$ sudo -i # mkdir /mnt/chroot # mount /dev/sda1 /mnt/chroot <--- replace sda1 for your system partition # mount -o bind /dev /mnt/chroot/dev # mount -o bind /sys /mnt/chroot/sys # mount -o bind /proc /mnt/chroot/proc # chroot /mnt/chroot # grub-install --recheck /dev/sda1 # update-grub
Then reboot and cross-fingers.