Assuming you have a 2D flappy bird-type game, you need to place Area2D nodes on both the bird and the tube.
1) Create Area2D nodes for your bird and tube objects
2) Create a new script for the tube.
3) Set up an
area_entered signal from the Node tab (next to the Inspector tab when you select an object in the editor). Connect the signal to the node that holds the script.
4) A new
_on_Area2D_area_entered(area) function appears in the previously created script. Put your logic here. The gist of it all is that when another Area2D collides with this object, you want to check what this other Area2D is, and if it is the bird, you restart the game. You do it somewhat like this:
if area.name == "Area2D":
get_tree().change_scene(MainScreen) # change to another scene