Random battery death for macbook pro?

So, to be fair, my computer isn't the newest model, roughly four years old. It's a 10.6.8 if it's helpful....

My battery was working just fine, but I changed states and went by plane. Obviously, being put in a carry on bag, my computer had to go through the X-ray stage in security. I didn't go on my computer right away after passing the gate, but once I got to my destination I tried logging in.... Only to find my computer wouldn't turn on. Weird. I had my computer on full charge when I left, so surely it wouldn't have drained within a few hours.

I get my mac plugged in.... and now I find that I suddenly have to replace my battery. It straight up won't charge the battery nor will it work without being attached to the wall the whole time. I would accept this change if my battery was showing signs of wear and tear but it wasn't. I'm scratching my head wondering what happened. Is this a normal thing for computers of its age? Was it a mistake to have my computer on carry on?

I'm stumped, am looking for a replacement battery. If someone could give me recommendations for it, I've found some third party options. Mostly I would like to know what could have possibly caused the battery to die. Thanks for your time in advance, everyone.