PC shuts down as soon as I plug in the power cord

So, I've got this Asus laptop.
Every single time I try to power it up, it just shuts
itself off after maybe three or four seconds, and it's doing this non-stop.
But get this—when the battery actually died,
it started working totally fine, and even showed
that the battery was at a full 100 percent. What the hell is going on?
Any idea what I should do about it?

Grab a multimeter and check the voltage coming out of your power brick. It should be sitting at 19.5 V—but honestly, don't take my word for it, just check the sticker on the adapter itself. See if the voltage is actually holding steady while it's trying to charge. There’s a good chance the charger just kicked the bucket.

I don't have a multimeter on me right now.
So, my laptop is an Asus Vivobook 15 X507MA—grabbed it about
a year and a half ago at Best Buy for around $633 bucks.
I haven't used it much, but hey, it’s actually working again.
I'm not sure if this is even related, but back when I reinstalled the OS about six months ago, it acted up a few times right after the install
—I remember it wouldn't even turn on, so I
ended up hunting online for some key combos to hold down, which finally got it to kick back to life.
Now, here's my thought: if the charger is the culprit, or if it's just totally fried, shouldn't the thing still power up since
my battery is sitting at a full 100%?
If I drag this thing down to a repair shop, they'd probably be able to tell me exactly what's going on, right?
Because if it's just the charger, that's an easy fix, but man, how am I supposed to know if the motherboard itself is toast?