- Whilst on the GRUB selection screen (where you select your operating
system, recovery mode, etc) use the arrow keys to navigate to the normal boot entry, bur rather than pressing enter, press the letter 'e'.
- Delete the line
- Press F10 to boot
If this doesn't work, try again, but first disconnect all USB devices (other than keyboard!) and extra monitors if you have them.
If you can not reboot, once booted, run the following, so that hopefully you won't have to do the steps above everytime you boot.