How can I access my iMac (high sierra) when password doesn’t work?

Upgraded HD that had Snow Leopard last night to High Sierra...but the password for that HD no longer works, and I can’t open High Sierra.

I have Yosemite and El Capitan on other HDs, and those passwords still work, but this drive, the original inside the iMac, is now inaccessible.

How can I bypass that locked startup screen?