Once merchandise is placed in the cart and a buyer clicks on the cart, the “Check Out” button/link is not taking them to the Check Out page to complete a purchase.
You’ll want to include a link to your store 
This is most commonly a theme problem, the button’s behaviour can be modified by apps or other customisations (like upsell apps) and if that app gets removed then the button breaks entirely. I worked in Shopify’s support for years and saw this all the time. Let me know if you’d like any direct assistance (or at least a pair of eyes that can try to spot the problem) and we can discuss!