Please try the following steps:

Quit Word, Outlook and EndNote first.

Go to the following location folder based on your Word program and right click on "WINWORD.exe" and select "Run as Administrator."

Word 2016: C:\Program Files (x86)\Microsoft Office\Office16\
For Office 365 (Word 2016), the folder will be: "C:\Program Files\Microsoft Office\root\Office16\
Word 2013: C:\Program Files (x86)\Microsoft Office\Office15\
Word 2010: C:\Program Files (x86)\Microsoft Office\Office14\
Word 2007: C:\Program Files (x86)\Microsoft Office\Office12\

Note: If Microsoft Office is not listed under "Program Files (x86)," look under just "Program Files."
Go to the File Menu and choose "Options."
Select "Add-Ins" from the list at the left.
At the bottom of the Add-Ins page, there is a "Manage:" dropdown list. Leave this on "COM Add-ins" and select Go.
On the COM Add-Ins page, Select the EndNote item and click the "Remove" button.
Next choose the "Add" button.
Browse to the following location and select the "EndNote CWYW.dll" file:  
EndNote X9: C:\Program Files (x86)\Common Files\ResearchSoft\Cwyw\19\
EndNote X8: C:\Program Files (x86)\Common Files\ResearchSoft\Cwyw\18\
EndNote X7: C:\Program Files (x86)\Common Files\ResearchSoft\Cwyw\17\

Note: If you have common files installed in a different location, you would need to find this CWYW folder in that other location.

Once you have Selected the "EndNote CWYW.dll" file, choose OK and OK again to close the COM Add-ins screen. The EndNote tools should now be in Word.

You would then Quit Word and reopen it normally to see if the EndNote tab appears automatically.