So I guess your game is over when the bird hits the Obstacle.
So now create "EVENT" Bird is in collision with another object (Pipe)
Create these Actions
Action 1: Stopping the Controls:
Organize all your events correctly. Now create all your control events together or in a group. (You can add groups by right clicking on the event and create group). Now lets say that you have created a group and all your control event in that group. When the bird hits the Pipe you can now deactivate the Group by SYSTEM --> Set Group Active --> Deactivated. So now your controls will stop working and Start new animation called "Fall" (where the bird is falling down).
Creating a Game Over Sheet: Create a Game over sprites in a separate layer. Create these event
Action - Set Layer (Game Over) visibility to "Visible".
Okay I know what you think. Isn't this layer visible from the beginning, Yes so create this above
Event - On start of layout - Action - Set Game Over Layer visibility "Invisible"
Hope this helps.