Thanks chaps, the reinstall Evernote suggestion fixed the Windows Outlook2016 add-in for me.
After upgrading to Outlook2016 a while back I had no "Save to Evernote" button on the ribbon. I finally got fed up enough to go find out what was wrong (leading me to this useful thread). I looked in the Manage COM Add-Ins and Outlook was telling me that the Evernote plugin was taking forever to start (it was on something like 2979 seconds when I looked!). So I fully quit Outlook and Evernote, UNinstalled Evernote, REinstalled Evernote (v5.9.6) and restarted Outlook. Didn't need to reboot. My Evernote button in Outlook is back.
FWIW I had just been using the Evernote auto-update installer so I was already on the latest v5.9.6 version, it just hadn't fixed the Outlook add-in after upgrading Outlook from 2013 to 2016, until I fully REinstalled Evernote.
HTH somebody. So glad I finally got that fixed. Really incredibly useful feature. Just wish it was possible on OSX Outlook2016 (I had to use an Automation script to support it here, and while it works well it isn't quite a smooth as the Windows version).