Black screen on startup | No startup sound | Caps lock light works.

I watched the video. I don't think that line is a big issue. I've seen on computers that went on to work normally, and I've seen it on computers with a VGA issue.

Going back to your laptop (it seems you have a mat screen or a high-res?), the 2011 is infamous for its VGA failures, so much so that Apple launched an extended warranty program for them and kept replacing the boards that fail some specific video tests for free until end of 2016.

I think your laptop has this disease, and unfortunately has showed it one month too late.

You will have confirmation of this issue returns.

Less likely, there is also the possibility that your laptop was updating firmware and bios, in which case it seems to hang on a black screen for a long time. If it happens again, leave it for some time. See if it evolves into a normal startup. I've seen it before, but with back-light and no display. It was installing the updates.

I've also seen it beep at start-up then "hang" for some time when installing SMC firmware. It also looked bad and some people tend to force shut down at this point. Don't. Give it some time.

In summary, I mostly suspect VGA issue and it will come back.

To a much lesser proportion, I suspect it was hanging on an update.

In all cases, keep back-up in case you have an SSD corruption from failed update - just in case, though that would be the least likely.