There are a few ways that you can get out of GNU GRUB and back into Ubuntu. One way is to simply reboot your computer and hold down the Shift key. This should bring up the GNU GRUB menu and you can then select Ubuntu from the list and boot into it.
Another way is to use the Boot Repair tool. This is a program that can fix many boot issues, including getting out of GNU GRUB. To use it, you will need to boot from a Ubuntu live CD or USB drive. Once you have done that, open a terminal and run the following commands:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
After boot-repair is installed, run it and select the "Recommended repair" option. This should fix the problem and get you back into Ubuntu.
If neither of these methods work, then you can try manually editing the GRUB configuration file. This should only be done if you are comfortable with editing text files and understand what you are doing. To do this, boot into a Ubuntu live CD or USB drive and open the file /etc/default/grub in a text editor. Find the line that says "GRUB_DEFAULT=0" and change it to "GRUB_DEFAULT=4". This will make Ubuntu the default option when you boot up. Save the file and exit the text editor.
Then, run the following command to update the GRUB configuration file:
sudo update-grub
Reboot your computer and you should now be able to boot directly into Ubuntu.
