Missing Notification Center Widgets & Share Extensions
Apple has a knowledge base article for this bug occasionally affecting macOS Yosemite, El Capitan, and now Sierra. This happens most often after installing an OS update.

To reregister all Notification Center widgets and Share Extensions, start a Terminal session, and enter the following:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -seed


(Triple-click the above line and press ⌘ cmdc to copy, then ⌘ cmdv inside Terminal.)