I’ve just seen a laptop that wouldn’t start up until the battery was removed. (It’s the second time I’ve seen this, so it’s definitely a thing.) I could see the laptop wasn’t completely dead as the battery light was on, but pressing the on / off button did nothing, whether the AC power was connected or not. Removing the battery (by sliding a catch that normally keeps it locked in place) allowed it to start and run Windows as normal.
I don’t claim to know why this happens. I just know that it can.
In today’s case, the owner of the laptop decided to just run it without a battery installed (obviously meaning it has to be permanently connected to AC power). In the previous instance I saw, a replacement battery made everything work as normal again, with or without AC power.
So now you know. If your laptop suddenly becomes completely unresponsive (and especially if it’s more than about four years old), try removing the battery.
By the way, be cautious when buying replacement laptop batteries. You don’t have to buy the original manufacturer’s battery, but some cheaper batteries can be a false economy (because of poor performance or a short life) or even dangerous. Only buy from a source you trust or that has been recommended to you.
Also, the Internet is full of articles and videos promising to tell you how to revive a dead laptop battery. I have no idea if any of these techniques work, but I suspect that if the battery is more than four or so years old any reprieve you win will probably be short-lived.