Hello,
When in your Ubuntu, open a terminal and type:
Code:
sudo nano /boot/grub/grub.cfg
that is if your grub is installed that way. Normally it'll be in that location. Once opened check if you find the string that gives you the error. Maybe something went wrong with the grub install but you would only notice it after the first reboot, hence why you could work the first day.
Kind regards,
Eric