Symptoms: rebooting immediately goes into the grub command line. The easiest way to fix this is to use a USB boot, and run tools to fix the boot. This page explains how to fix it from the command line.
The following commands are how I fixed it.
grub> set root=/dev/sda2
grub> linux /boot/vmlinuz.old root=/dev/sda2
grub> initrd /boot/initrd.img.old
grub> boot
That will boot the system into the previous kernel, which probably works. Presumably, the current kernel fails.
Once it boots, open a terminal window and do the following:
sudo /sbin/update-grub
That will produce a fresh /boot/grub/grub.cfg file.
Reboot the computer.
Explanations
“set root=” isn’t strictly necessary, but I do it.
“linux” sets a few things for the linux boot. The first parameter specifies the kernel. The “root=” sets the root directory during boot. You can use the UUID as well, but in my case, this is easier to type, and it’s how it works.
“initrd” sets the initial ramdisk. This contains drivers to load the rest of the system. If you don’t specify a ramdisk, you can’t continue the boot. (I think you will be dropped into BusyBox.)
Once you’re in the OS, you can run update-grub to fix the config file and run a lot of other programs that check things out.
