Stuck on lock screen

MacBook Pro A1278 stuck on Lock Screen. When I installed El Capitan I did not set a password nor did I register the OS/laptop to to my account - just wanted to see if the thing would work. It did work and I used it for about a week. I even upgraded the RAM to 16GB and it it kept working for another four days. Then one morning it would boot - I can hear the startup "ding" and the touchpad and/or a mouse works - to a Lock image on the screen and a field presumably asking for a password. Well, I never set a password so no matter what I enter, it doesn't start. I've tried all of keyboard combinations suggested: hold power button during startup, hold Option during startup, Cmd-R, Cmd-Option-R, Cmd-Option-P, CmDs-Option-P-R and a few others that I've seen suggested on the internet. None of them work. I can't boot into a USB memory stick either - I tried memory sticks with a bootable Linux distro, but the machine will not recognise it and stays on the screen that displays the lock. Oh, and I've also tried entering the password for the Linux install. tried entering it three times in a row to display the login/recovery options as well as the password ford for the machine on which I created the bootable install. Then I did all of the above again with a supposedly bootable El Capitan USB install memory stick. The A1278 was working with El Capita before this. I'd now prefer to install Linux Mint

What am I missing?