startup progress bar goes half way and then shuts down
On startup the Apple logo appears and then the progress bar. It starts and about half way through it shuts down
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Attempt to restart in Safe Mode, instructions are in Use safe mode to isolate issues with your Mac - Apple Support and then restart normally. It that does not work attempt to reset the NVRAM per How to reset NVRAM on your Mac - Apple Support
If the NVRAM reset doesn't work, restart in Recovery Partition (Command + R on startup) and run First Aid in Disk Utility. If it still won't boot you have a hardware issue likely and need to take it in for service at your local Apple Store or AASP.
Good luck!