I don't know what exactly caused this since I didn't notice it until after performing a core and multiple module updates (all to current versions) along with removing the default kickstart products etc, but the add to cart button totally disappeared everywhere on the site including all display nodes and all views.

In views that used the add to cart field, I now get 'Broken/missing handler' and the Relationship is also missing. I added the relationship back (Content:Product) but there is no Field to select for the "Commerce Product: Add to Cart Form"

Also: In the display nodes where multiple products used to display in a select list, they now appear without a select list, just as line items and no 'add to cart' button.

Note: after the updates all my cart modules where disable and I had to enable them again. Perhaps something else is disable that I do not see yet.

I would greatly appreciate it if anyone has an idea on how to fix this.