MacBook Pro dead and unresponsive

Hi.

MacBook Pro 2012 i7 2.6

https://everymac.com/systems/apple/macbo...

*Dead- won't boot up. Or respond to long power button push.

*Green MagSafe light indicates battery is charged. (could it be defective anyhow?)

How do I figure out if it is the power supply, the logic board, battery, or something else? Without buying a part and installing it, one thing at a time.

Hope someone has the experience to point me in the right direction.