EDIT: Everything seems to be working now. Must have been an issue on SquareSpace's side.
I tried to edit a specific product by entering a Facebook "Share" button code, but I did so incorrectly and now I am unable to edit the product itself (keeps showing spinning square) and the inventory for my products has disappeared.
The code I entered into the "additional details" with a code block was:
<div class="fb-share-button"
data-href="https://www.your-domain.com/your-page.html"
data-layout="button_count">
</div>
...Which is obviously incorrect.
I am on version 7.1.