Safari created a strong password but did not remember it
Sadly this is a problem that happens with certain websites, and sometimes due to a small hick-up. (of you followed some of the links on google they all have there reasons, Im sure I don't have to repeat them here)

Why they haven't got a feature (like Dashlane) to show you the recently created but not saved passwords is a mystery to me.
AFAIK you'll have to reset your password.