Start by checking status.mechanic.dev and shopifystatus.com for service interruptions. (If you're not signed up for Mechanic status notifications, this is a good time to take care of that!)
If the event is well and truly missing, email [email protected] for further assistance.
β οΈ Please note: these investigations usually conclude that Shopify has simply failed to deliver that specific event webhook. Shopify does not offer a strict guarantee on webhook delivery (see their documentation), which means that the solution is usually to ensure tasks can recover when events go missing.