The screen will change when you take damage from enemy attacks. Damage heals naturally over time while at rest, but take too much consecutive damage and you will die, resulting in the Game Over screen.
Critical Wounds and First Aid
Depending on your distance from the enemy when attacked and the weapon that was used, you may sustain a critical wound that can limit your actions and prevent automatic recovery.
Withdraw to a safe place, then perform first aid.
First Aid icon (The Phantom Pain)
First Aid icon (Ground Zeroes)
The Game Over screen appears in various situations, such as when your character dies or you fail a mission.