Deleted symlink to /etc and now missing sudoers
Boot to Recovery Mode (cmd-R)
Open Terminal from the menubar Utilities > Terminal
Soft-link /private/etc to /etc on your main Volume:

ln -s private/etc /Volumes/Macintosh\ HD/etc 


Replace Macintosh\ HD with the name of your main volume (if your forgot it, just enter df or ls /Volumes/ and check for its name! Don't forget to escape blanks with a backslash.)