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When I hold the left shift key on grub menu, nothing appears and it still boots to Ubuntu like it normally does (Ubuntu is the only machine installed). How can I change the password for root elseway? I have access to the terminal but when I write sudo, it says that I am not a sudoer and incident will be reported. I was a suoder and used to be able sudo until today which I messed up with system. Apparently the solution is changing the root password.enter image description here

When I am trying to start the GUI it tells "Failed to start the session" however I used CTL+ALT+F1 and have access to terminal.

Note that I am not a sudoer and cannot change grub settings.

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marked as duplicate by muru, WinEunuuchs2Unix, David Foerster, Anwar, Eric Carvalho Nov 8 '16 at 11:13

This question was marked as an exact duplicate of an existing question.
    
Do you know the current root password? Is it set or not? There is a difference if you want to change a known or unknown root password. For known root password type su and you will get root privileges if you know root password. If you don't know it, you should boot from a USB or CD/DVD, mount the Ubuntu partition and change the password. – nobody Nov 7 '16 at 20:39
    
I am not sure "Failed to start the session" is linked to sudoers. You must have messed something else too. – nobody Nov 7 '16 at 20:41
    
The error "Failed to start session" existed before I mess up with the sudoers list and remove myself and everyone else in our group as a sudoer. – Mona Jalal Nov 7 '16 at 20:42
    
Boot from live CD or USB, mount your root partition and fix the sudoers file – Zanna Nov 7 '16 at 21:16
    
From the answers to the linked question use one of the methods that don't rely on the recovery mode . – David Foerster Nov 8 '16 at 4:06