Shopify Cart Button Repair

I am currently seeking the expertise of a Shopify proficient professional. My ecommerce site has a persistent issue; the 'add to cart' button is not functionally properly as when I go to click it, it comes up with a red circle with a line through it. I have the button used in multiple places in my site such as 2 times on the product pages and on my sticky add to cart. Each ATC button doesn't work. However the buy it now button works. This is on an external paid theme for Shopify. This is a site-wide problem, impacting all products. The error came about spontaneously without any recent changes to the code or theme.

The ideal freelancer has:
- Proficiency in Shopify, with experience troubleshooting cart issues
- Ability to work quickly and efficiently, as this problem is directly affecting sales.

Your task includes:
- A comprehensive diagnostic to determine the issue
- Swift implementation of the necessary fix
- Additional recommendations to prevent future issues if relevant
- Quality assurance testing post-implementation to confirm the problem is rectified.

If you're a proficient Shopify expert with a knack for swift, efficient troubleshooting, I'm looking forward to your bid.