Fixing /etc/sudoers file
If you have access to finder and a system administrator account, you can fix this without a reboot by leveraging these.

Open up your finder and go to /etc. In here, you can view the details of the sudoers file, including permissions - screenshot below. 



Change the permissions for "everyone" here to Read &amp; Write.

At this time, you can edit yourself back into the file with any text editor. You will have to return permissions for "everyone" back to No Access before sudo will allow you to perform a sudo.