They only show up when a program using these controls - say the music player - is running in the background.
Originally Posted by corsica
When you turn on the device, they will be on the screen for a few seconds and then disappear. In this case you get them back by pushing once the volume control (either direction will do the trick). This also works when the music is paused and the controls aren't on the lockscreen.
If they don't show even though music is playing, I would restart the device and hope for the best :)