First, try starting up in Safe Mode. Turn on your Mac, then immediately hold up the Shift key until your Mac boots up. If your Mac starts up, try restarting normally. How to use safe mode on your Mac - Apple Support
If the issue persists after this, start up into Safe Mode again, and check your startup items to make sure that something nasty isn't booting up at the same time as your Mac. If your Mac has a blue screen or other startup problems - Apple Support
If this doesn't solve the issue, or your Mac doesn't boot up into Safe Mode, you'll need to reinstall macOS. Turn off your Mac, then turn it back on again. While your Mac is starting up, hold Command-R. After unlocking your disk, click Reinstall macOS. This won't touch any of your personal files, just macOS itself. How to reinstall macOS from macOS Recovery - Apple Support
If, for some reason, this still does not resolve the issue, you'll need to erase your disk and reinstall macOS completely. Start up into macOS Recovery, then click Disk Utility. Select the disk you want to erase, then click Erase. When asked, format the disk as APFS, and use the GUID Partition Scheme. How to erase a disk for Mac - Apple Support
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If your Mac doesn't start up all the way - Apple Support