A collision with two quantum filaments in 2368 that crippled the Enterprise-D, cut its sections off from one another and left Counsellor Troi as the senior officer on the bridge.
The quantum filament collision is the event of the Next Generation episode Disaster. The strikes knock out most of the ship's systems and kill the officer on watch, so Deanna Troi takes command with only Ro Laren and Miles O'Brien to advise her, and has to decide whether to separate the saucer from a failing antimatter containment system.
Elsewhere Picard is trapped in a turbolift with a broken ankle and three frightened children, whom he leads up the shaft. Worf delivers Keiko O'Brien's baby in Ten Forward. Beverly Crusher and Geordi La Forge put out a plasma fire in a cargo bay by opening it to space.
Riker and Data reach engineering through the service crawlways, where Riker has to remove Data's head and connect it to the controls. Troi's decision to wait and send power to engineering gives them the chance to restore containment and save the ship.