Mac won't turn on

Hi all.

I am a Macbook pro afficionado who is at my wits end with the unexpected development of my mac suddenly switching off. Battery indicator is showing life and the charger is indicating green when the battery is not full. The engine is virtually cold. I am not sure what the likely cause could be or if there is a remedy for it DIY style.

A friend of mine has a similar problem but his was a development after a Sierra Public Beta installation. The charger reflects orange if the battery is not full but the computer does not turn on anymore. Help?