If the 'insert citation' icon in Endnote is greyed out and no longer clickable, this usually means the Cite While You Write tool in Word has been disabled or that Word is linked to Endnote Online rather than Endnote Desktop.
To enable the Endnote tab in Word:
In Word, check that you have the 'Endnote' tab in the ribbon menu on the top of the screen.
If not, follow these instructions to add it back in: https://libhelp.ncl.ac.uk/faq/43291
With Word 2016, the Ribbon tab may be unchecked so you will also need to:
To link Word to Endnote Desktop:
If you have been using Endnote Online it may be that Word is currently still linked to Endnote Online rather than Endnote Desktop.
To fix this: