macbook won't coplete startup
macbook tries to start up but doesn't finish startup
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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