You’re in grave danger—mark 1-fatigue every few seconds (or each exchange) until you free yourself.
Doomed is the status for when you’re drowning, or when you’re on fire, or when the stone around you isn’t simply immobilizing you—it’s crushing you. It represents a constant, ongoing pressure upon you, causing you to mark fatigue at a steady rate until you are free. The GM decides exactly how often you mark fatigue during play, unless you’re in exchanges—then, you should mark 1-fatigue at the beginning of each exchange. Remember that if you can’t mark more fatigue, you mark conditions instead.
Much of the time, you can’t be Doomed unless you’re first Impaired, or Impaired and Trapped. You won’t be drowning unless something holds you in the water; you won’t be slowly crushed by rock without first being trapped in rock. But sometimes it’s appropriate—your entire outfit, head to toe, can be set on fire by an angry firebending master without warning!