Ok...stupid question.

When looking at my timeline, there is a playhead/line with an upside down triangle on it...that USE TO scroll with the video as it played. It stopped scrolling.

The video plays fine when I hit the spacebar, but that playhead/line thingy doesn't move along with the video. What did I turn off?

What did I turn off? Probably nothing. You switched from Canvas to Viewer (the other window which shows that upsidedown triangle in the window under the picture).
Click on Canvas, hit the spacebar and there it moves again :-)