Big Sur

The drum icon is a user profile icon. Are you able to click the drum and type in your Mac password? If you had setup automatic login previously and you do not remember your password, it should prompt you to reset the password after a few attempts and failures. In most cases you will require your Apple ID / PW for iCloud to reset your password. If you enabled FileVault2 disk encryption it should still work as the recovery key should be stored in iCloud.

So you have a 2012 iMac correct?

CMD + R should have worked.

1. Power-off the Mac completely, you may have to hold the power button for 6+ seconds. If that doesn't turn it off unplug it and plug it back in.
2. While holding CMD + R press and release the power button keep holding CMD + R
3. It should boot into Recovery Mode where you have an icon to re-install macOS.

Is it asking about reseting the Keychain? Is that what you mean? If you reset your password, then the Keychain's password is the old one. If you know the old password you can enter it to unlock the encrypted keychain which holds all your saved passwords, certificates, etc. If you do not know the old password, then you can reset the keychain and it will be renamed and a new keychain created using the current new password. If you use iCloud Keychain, those passwords should not be impacted.

Do you have a Time Machine backup? I would install Big Sur again from Recovery after deleting the internal Macintosh HD. That would be a clean installation. Then restore your data from backup. It’s likely you had something installed that is not compatible with Big Sur.

Something went wrong with the upgrade. Boot the Mac into Recovery Mode by holding CMD+R while powering on. Then re-install Big Sur on top of your existing installation.

Still cannot get into Recovery Mode via CMD + R?