When tracked stock runs out, customers cannot buy the product. Its page stays visible in your shop, marked as out of stock.
To find what ran out, open Inventory and set Stock status to Out of stock, or check the alerts in your notification bell.
When new stock arrives, press Adjust stock on the product's line in Inventory to record it, or open the product's own stock page and record it as Received. If you track sizes or colours separately, each one has its own line, so update the one that arrived.
If you chose No tracking for the product, sales never stop because a count runs out. Switch the product off in Products when you cannot supply it.