The odds of this occuring are so astronomically high as to cause little-to-no concern, but there is a random chance that in an instant, the entire universe will simply cease to exist. Everything, you and me, stars and planets, entire galaxies would vanish, leaving behind nothing to indicate they ever were there. There wouldn't be- nay, couldn't be any defense or precautions against this, because there's no telling when it would happen. You could be studying, having fun with your friends, or working when it happens. No warnings to the oncoming oblivion.
Why would the universe just vanish like that? After all, shouldn't there be something or some things to stop that? In theory, yes. But we have barely explored our own galaxy, and there's no telling what could be responsible for such an event. Perhaps the cause of this apocalypse could be from outside our universe, like a malevolent force.
What would you do, knowing that everything and anything could wink out?