How to get to root after switched to new user without being in ubuntu's sudoers

Question

This is frustrating when I added a new user, then su newuser, couldn't go back to root by all means.
It says not in sudoers afterwards. I've no root password, neither able to login from putty, it said login as ubuntu which rejected key, but i can login as newuser into newuser account. I sent 3 emails to support, no response.
Hope I can do something here since my website is down for days.

Answer

Try to fix the files by attaching the volume to another running instance. Basically, you stop the instance, detach the boot volume, attach the volume to the running instance, mount it to the OS, the fix the sudoers or /etc files. Then do the reverse. Also, you can attempt to connect to the Serial Console to fix the files.

How to get to root after switched to new user without being in ubuntu's sudoers

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