I can't remember my iMac password and I can't reboot
How do I reset my password on an 2010 iMac?
How do I reset my password on an 2010 iMac?
Forgot Your Account Password - Change or reset the password of a macOS user account - Apple Support.
Forgot Your Account Password - Change or reset the password of a macOS user account - Apple Support.
I can't get past the login page
Did you actually read the document linked to above?
I can't remember my iMac password and I can't reboot