Notes not opening on OSX El Capitan 10.11.16 on MacBook Pro Notes will not open either from dock icon or from Finder, even after a forced quit and re-starting of MacBook Pro...just begun happening today during normal MacBook usage.

Suggestions?
Go to System Preferences -> Internet Accounts and see if you have any old email accounts there. If you do, then delete them.

Also, you could go to System Preferences -> iCloud, and try to switch Notes off and then on again.