Press and hold the Menu and Center buttons for a few seconds until the apple shows on the screen and lights up. (top of scroll wheel and center button).
If it continually shows an apple but doesn’t quite light up, your battery is probably flat. This is good! Just recharge for awhile and consider trying a dedicated power adapter instead of a USB port. This has happened to me more than once where the USB port could not supply enough power to charge the ipod.