What does a non-booting macbook displaying a lightning bolt mean? I returned to my MacBook after a few days of not using it, and there was a message that the system had run out of application memory (weird...). So, I followed the recommendation and Force Quit a bunch of applications. In order to just "start fresh", I decided to kill all applications and simply restart the whole machine.

Now that I've killed the applications and restarted the computer, it won't boot. It simply displays a gray circle with a white lightning bolt on it, moving around the screen, screensaver-style. If I unplug the AC power cord, a battery indicator is displayed at the bottom of the screen.

I attempted an SMC reset, but that doesn't help. What's going on, and what should I do?
That looks like the Thunderbolt logo. Your Mac probably thinks that is in Target Disk Mode.