My Macbook Pro won’t start up

Try an SMC reset.

Check your power adapter.

https://support.apple.com/en-us/HT203207

or

https://support.apple.com/en-us/HT204652

Take your laptop to Apple or an Apple Authorized Service Provider to be serviced.

If your Battery is dead you will not be able to start unless you are plugged into the mains.

Once you get it booted up, check your battery condition, holding the option key and click the battery icon in the upper right menu bar. Anything other then Condition: Normal needs to be serviced.

About Mac notebook batteries - Apple Support

https://support.apple.com/en-us/HT204054

Determine battery cycle count for Mac notebooks - Apple Support

https://support.apple.com/en-us/HT201585

Batteries - Maximizing Performance - Apple

https://www.apple.com/batteries/maximizing-performance/