I've just bought an iMac, reinstalled Yosemite, put in my details but, coming back after a short pause, found it had gone to sleep and wouldn't accept my password to wake up. I'm sure I put in the right one. I've seen advice that re-booting to Recovery might offer a way of recapturing my password, or changing it to a new one, and tried it again and again. But whether I restart holding down Command+R, or holding down the Option key and then choosing to boot from Recovery, I just can't find any options to click that would lead me to either my password or any way to put in a new one.
I'd be grateful for help.