Mac will not start up

Try an SMC reset:

How to reset the System Management Controller (SMC) on your Mac - Apple Support

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

Try Single User Mode (hold command-s keys)

At the prompt, enter this command:

/sbin/fsck -fy

Press the Return key.

This will check your directory and report that it was OK or repaired. Then enter this command to startup normally:

reboot

Run Apple Diagnostics  

Press and hold the d key on startup:

https://support.apple.com/en-us/HT202731

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

Genius Bar Reservation and Apple Support Options - Apple

Please try resetting the NVRAM and restarting in Safe Mode (Safe Mode can take up to 10 minutes to startup), Instructions for each are in the links below.

https://support.apple.com/en-us/HT204063

https://support.apple.com/en-us/HT201262

If neither of these work, please let us know and we can try some other troubleshooting steps.

If you don't have a complete and recent backup done in the last few days and unless you're very familiar with recovery partitions and troubleshooting in general, I suggest you take it to an Apple Authorized Service Provider (AASP). The chances of you making a rookie mistake and erasing your stuff are fairly high. Apple Store will not backup your stuff. If there are no AASPs close enough to you find a competent independent repair shop. Almost every town has one.

The fact that the Mac actually starts up and begin loading is good news as it means the startup drive is functional.

Once your Mac is fixed, follow a strict backup routine with TimeMachine. Please.