What if a product is unavailable?

Mark it sold out. Go to Products, tap the three dots on that product, and select Sold out.

The product stays in your store and customers can still see it. They just cannot order it until you switch it back.

Changing it rather than hiding it

If the product needs a fix rather than a pause, see How do I edit, hide or remove a product?

Switching it back on

Same route. Products > three dots > select the option to make it active again.

Do it before the order, not after

This is the small discipline that prevents most seller problems. If you know on Monday that you cannot get good mangoes this week, mark the mango cake sold out on Monday. It takes ten seconds and it saves a customer the disappointment of ordering something you then have to reject.

If someone has already ordered it

That is a different situation and a more awkward one. See What if I cannot make an item that is already ordered?.

If it is everything, not one thing

Pause your store instead.