iMac reboots after entering password

Thank you for your post.

If you have any antivirus software installed, then remove it A.S.A.P. as they interfere with the system operations and performance.

If you don’t have anything like that installed, then try the following:

First, let’s see if this issue happens in Safe Mode:

1. Shut down your Mac.
2. Press and hold the power button on your Mac. As you continue to hold the power button, your Mac turns on and loads startup options. When you see Options, release the power button.
3. Select your startup disk. It's named Macintosh HD, unless you changed its name.
4. Press and hold the Shift key, then click the “Continue in Safe Mode” button below your startup disk.
5. Log in to your Mac. You might be asked to log in again.

If the strange behaviors do not occur, then it may possibly be a software issue.

If the issue doesn't continue in safe mode:

Leave safe mode by restarting your Mac normally, then try to reproduce the issue.

If the issue doesn't return, it might have been resolved by the other things that safe mode did at startup, such as checking your disk and clearing caches.

If the problem persists, then let’s try this:

• Software Update.
• Use Apple Diagnostics to test your Mac.

Need more help?

• United States: 1-800-APLCARE.
• Contact Support.
• Mac Support.
• Mac User Guide.

Good luck! 👋🏼😉

Smiliñ 😎 Brian

Try to start your Mac in Safe Mode: Start up your Mac in safe mode - Apple Support. Wait 5 minutes to let macOS finish some housekeeping and reboot in normal mode.

6x6 wrote:

Try to start your Mac in Safe Mode: Start up your Mac in safe mode - Apple Support. Wait 5 minutes to let macOS finish some housekeeping and reboot in normal mode.

Also try booting normally without any external devices plugged in if you have any external devices.