Why won't my MacBook Pro turn on?

Hey there. My MacBook won't turn on whatsoever, the last time I had used it was about a month ago and it died naturally due to low battery, but hasn't turned back on since. I hadn't tried to turn it on for a couple of weeks after it died, not sure if that has any effect?

I bought a tech tool kit and followed a YouTube video to open up the laptop and troubleshot. Disconnected/reconnected the hard drive, disconnected/reconnected the battery and neither worked. The video led me to believe it could either be the battery or the motherboard itself. Though perhaps someone on here has more insight?

When I plug the laptop's charger in, the light is green but will just stay green even after hours of it being plugged in. Does this indicate that the battery is okay, if it's still showing green?

I'd like to try to fix it myself if possible, but don't want to spend money on a new component and wait to receive it, only to find out that wasn't the issue.

If anyone has any advice, further troubleshooting options, etc then please let me know! Thank you all so much.