Apple - Character Accent Menu doesn't appear anymore in macOS Sierra

Found the solution. Go to System Preferences > Keyboard > Keyboard tab > Modifier Keys(bottom right corner). Now press restore defaults and see if that works.

I got the same issue after a recent update now, found the answer here and it worked for me. In Terminal type

defaults write -g ApplePressAndHoldEnabled -bool true

and then log out and log in again, this should fix the issue.