Inspired by my last post, but somehow messed up your sudoers file?
Don’t be afraid there’s a solution
So your
$ sudo visudo
is throwing something like this at you
>>> /etc/sudoers: syntax error near line 28 <<< sudo: parse error in /etc/sudoers near line 28 sudo: no valid sudoers sources found, quitting
Fear not, just type su and then
visudo
And then you can delete the line that’s causing the error (in this case line 28) or try to correct it.