Just to add to Erfan's answer:

If there were no products assigned to category "before", there would be no products listed under "Category Products" tab.

Reset the Filter - is quite important in case like this - and very easy to abandon, while seeing blank list (no products) at first glance.

Addition to Erfan's answer:

If there had been no products assigned to category before, there would be no products at all listed under "Category Products" tab.

Reset the Filter - clicking it is a must even when you see blank list (no products assigned). It will then show all products in whole shop.