I do know my password, but your suggestion gave me an idea which appears to have led to a solution, so, thanks. I had logged in as root but it only kept me on the terminal. This time I tried opening with Recovery Mode via Advanced Options, and that got me a display and some kind of pseudo lxde system in which there was no openbox root menu. The complete lxde-core is not installed but a number of lxde components are. So I removed lightdm and lightdm-gtk-greeter and re-installed them without much gain. Similarly for changing passwords. Next I installed Slim, made it the default display manager and successfully opened the pseudo-lxde system as an ordinary user, but it identified, but wouldn't open, the openbox session. Finally I re-installed openbox, and that seemed to get everything back to normal except with slim as display manager, which is ok with me as I like slim better than lightdm. Now to see what tomorrow brings.