MacBook Pro won't start

Try an SMC reset:

https://support.apple.com/en-us/HT201295

Try starting up in Safe Mode:

Use safe mode to isolate issues with your Mac - Apple Support

If neither of those work boot into Recovery and Reinstall Mac OS.

About macOS Recovery - Apple Support

If none of those work make a Genius Bar appointment for a free diagnostic:

Genius Bar Reservation and Apple Support Options - Apple

Try holding the Option key and then press the Power button (while the mac book pro is off) and see if it boots there.