Regarding the below (-----) info from another post, my system will try (as is said below) to hibernate, but it won't get all the way there. Hard drive activity stops, but it does not shut down. If I turn the power off, then back on, it will start, but I have to "restore previous session" in the browser and other open apps don't restart, with the accompanying hassle.

So because it tries, will the suggested fix then cause it to work correctly, or is the literal "try" not really what he means?!?

PLEASE NOTE - this is a desktop system, not a laptop.

Before enabling hibernation, please try to test whether it works correctly by running pm-hibernate in a terminal. The system will try to hibernate. If you are able to start the system again then you are more or less safe to add an override.

To do so, start editing

sudo nano /etc/polkit-1/localauthority/50-local.d/com.ubuntu.enable-hibernate.pkla

Fill it with this

[Re-enable hibernate by default] Identity=unix-user:* Action=org.freedesktop.upower.hibernate ResultActive=yes

Save by pressing Ctrl-O and exit nano by pressing Ctrl-X

Restart and hibernation is back!