If a customer lands on an out-of-stock product, there's a good chance of them leaving your store entirely. Especially if they came from an organic method like Google.
If a product's out of stock and you know you won't get any back soon, add a message to its product description. Explain it's out of stock, when you might get more, and give customer an alternative product or two they can order instead.
This can help rescue sales that you would have otherwise lost.
You could also edit your theme code and add that sort of message in there, but it's risky and will take some know-how to match up products automatically. It's a good idea to have a developer add in the slower season.
The quick-and-dirty way is to just edit the product description. Just remember to remove that edit once it's back in stock.
Are your repeat customers defecting? Use Repeat Customer Insights to find out where in their lifecycle you're losing them and what you can do to win them back.