When you put an item in your cart, it can still sell out. This is for a number of reasons, but what it means for you is that it's best to not wait too long before you finalise your purchase.
Will it come back?
It might! Even when a product is sold out, it may come back into stock: throughout the day, (unpaid) orders are cancelled and their contents are put back in inventory.
So don't despair, but grab a chair.
And then come back. To check the stack.
Maybe we shouldn't be rhyming.