category or product listing problem

I having an issue and I know it's probably something I'm doing wrong, but here I go. When I select the store from a menu it list the categories and you can click on the category and it will go to another page and list all the products. that works great. but if I click on the menu selection to access the shopping cart , I get "The Cart is empty" which works. no problem so far. below the message there list the categories again. when I click on one of the categories, I go to screen that has the header, footer etc. but no product list. the url are the same both, just when I'm going to it from another selection on menu it doesn't list products. What am I doing wrong?