I don't see where you add the panel to the frame. I would guess you need code like the following:
//frame.setSize(500, 500);
frame.add(windowContent);
frame.pack();
Also, the entire structure of you code is wrong. You should NOT be using static methods and variables. I suggest you read the section from the Swing tutorial on How to Use Card Layout for a working example with a card layout and so see how to better structure your program.
Take a look at other demo programs from other sections as well, for example How to Use Labels
because the demo there show a different structure for you code by extending a JPanel which may even be easier to follow.