How to boot with a previous kernel. This can be useful if a system fails to boot after a bad kernel update.
1 Boot into GRUB
Turn on the laptop and tap the esc key repeatedly.
Stop pressing the esc key when the star logo disappears.
This should present you with the below menu:
2 Too many ESC's
If you see the below prompt, you have gone too far.
Once at the grub command line, type normal, hit Enter, and then tap Esc until the menu is displayed.
If not, restart your laptop and repeat the first step.
3 Select the kernel to boot
Use the arrow keys to select Advanced options for Ubuntu.
You will be presented with the below menu. Use the arrow keys to select a different kernel.
Generally when troubleshooting, it is best to select the oldest version of the kernel - this will be at the bottom of the list without "(recovery mode)" on the end.
4 Boot your selection
Press enter to start the laptop with the selected kernel.