Follow these steps if you no longer have the ability to insert your letterhead:
Open Microsoft Word and then click File.
Click Options and then click Add-ins.
Click on the drop-down menu that says Manage.
Select Word Add-ins and then click Go.
Tick the unticked box in the Checked items are currently loaded area.
close Microsoft word and reopen it
📌Note: The add-in should then appear on the top bar. If it doesn't, try the steps a second time.