Log into Shopify Admin and go to the Online Store section
On your current active theme, click Customize
Look for Product Page Settings
In the left sidebar, navigate to Product pages or Product template
Look for a section that controls Related Products or Recommended Products. It could also be under a different name depending on your theme (e.g., Upsell Products, Product Recommendations, etc.).
Turn it back on if you find a toggle or option to enable this feature. It may be disabled or hidden.