So, if you put code like this on the player node, you can read the y value of position and that will tell you how far they have fallen. This is in the 2D case, but should also work for 3D if you change the logic a bit (I believe y goes up in 3D, so you'd probably check if it was less that some value, e.g. position.y < -10.0). In this case my one scene is called "Root.tscn". This loads the same scene we are in, which is a simple way to reset things back to the beginning.
if position.y > 600:
get_tree().change_scene("res://Root.tscn")
In a simple tutorial like this, you can save the player's initial position and then restore it when they fall. For example, at the top of the script with your globals, add this:
onready var initial_pos = position
Then later in _process check the y value and reset the position:
if position.y > 600:
position = initial_pos
Hope that helps.
