Is your add-in missing? Learn how to re-enable it with this step-by-step tutorial
Excel Add-ins can get disabled when Excel crashes. There is a Disabled Items window on the Excel Options, Add-ins menu that will allow you to re-enable or reactivate the add-ins that have been disabled.
Checkout my video on how to install an Excel add-in:
I also have an article with a step-by-step guide on how to install add-ins.
If this does NOT resolve the problem for you, then you might be impacted by an Office Security update that was released in July 2016 by Microsoft. Checkout this article for more details and instructions on how to fix it.