Cant log in to my imac

Hi,

Following tip could works for your issue.

1. Restart your Mac. ... 
2. While it is restarting, press and hold the Command + R keys until you see the Apple logo. ... 
3. Go to the Apple Menu at the top and click Utilities. ... 
4. Then click Terminal.
5. Type “resetpassword” in the terminal window. ... 
6. Then hit Enter. ... 
7. Type your password and a hint. ... 
8. Finally, click Restart.

https://support.apple.com/en-us/HT202860