My MacBook Pro isn't turning on, at first it would only turn on when being plugged in by its charger and continued to stay on 0% and now it won't even turn on even when plugged into charger. What's wrong with it?

Troubleshooting a Mac that won't turn on...

Is this an early 2008 MacBook Pro?

I observed two of these machines work perfectly for several months (different users and workflows) with no issues and with normal shutdown. The next press of the power button is greeted by about 3 - 5 seconds of DVD click, hard disk seek, and then nothing. Different batteries, chargers, and memory chips were used with no machine revival.

As these two machines were fully depreciated employer hardware, they were scrapped, rather than provided to Apple for hardware diagnostic and likely expensive repair.