Desktop screen freezes after unlock

Startup in Safe Mode and see if it will allow you to login.

Start up your Mac in safe mode - Apple Support

Then Restart the iMac Normally and see if you can login.

Thank you for sharing your concerns. 😉

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

You're welcome. Please let us know if that helps or not.