You chose to delete one or more of your product lines and suddenly the products underneath disappear. Don't panic! You didn't delete them. You simply removed your user permissions inadvertently.
When setting your permissions, you can easily add permissions such as "product owner" to a product line and then it's inherited to the products below as well. It's very easy. But if you then delete that product line, you also remove those permissions and you lose access to those products.
Go into your User permissions under Account settings and add yourself as a product owner to those missing products (I promise, they are in the list). Now you will see them again in your drop-downs.