I don't suppose the little slider on the usb stick has accidentally been moved protecting the contents?
Have you had a look at what is on your usb stick? Can you see if all of your music has been put into the same folder or have two folders been created? If two folders have been created then it is possible that the player in your car is only recognising one.
When you have ripped your cd's to WMP do you then transfer them to your stick by clicking on "my computer" and double clicking on the usb icon and leaving this open - then select "my music" then dragging and dropping this folder onto your usb stick in my computer?