I just found out all my product pages "Add to Cart" buttons were disappeared. Before is fine, after I make a new collection for BFCM and adjusted the price then all buttons gone...any help would be greatly appreciated!
In this case your "Add to card" button is hidden by the script - special-offers.js (I think it is the code from some cross sell app).
I suggest you try to enable/disable all the apps which are installed in your store. Perhaps, one of these apps works incorrectly or conflicts with another app.
Hope it helps.