You mean ruptured? Well, there would be an earthquake of at least M8.6 followed by a tsunami. Tens of thousands could possibly die, thousands would go missing, and millions would be without power. It would take a week or more before ground relief could come in due to all major roadways being out of commission.
Recovery would take years, and some areas would collapse economically, while others suddenly thrive due to an influx of survivors wanting to start over. The populations of western Washington and Oregon would decline significantly, with the eastern portions of those states seeing an increase in population, along with Idaho, Montana, Nevada and Utah. Northern California's population will shift slightly east and south, but will only decline if the state government refuses to allow quake survivors to pick themselves up and instead imposes stifling regulations on them that keep them impoverished and homeless.
Within a few months, activity at Mt. St. Helens and/or Mt. Rainier would increase. A year or so later, one or both would have minor ash eruptions. If this happens on Rainier, it could indicate a more severe eruption building. But this won't matter as most of the towns west of it will be rendered ghost towns by the tsunami.