Comment 14 for bug 973778

Just to update-

My installation was done as regular Ubuntu, using Unity. I had later added Xubuntu Desktop package, as Unity was too resource hungry on this machine. (AMD dual core 11.9ghz, 4gb DDR2, 32gb ssd).

This issue started manifesting probably a couple days ago, and I recall seeing a unity update prior to this occurring.

Anyway, this evening I removed Unity, gnome-power-manager and gnome-session components related to power, and also removed indicator-power (which also triggered a few other auto-removals related to the gnome/unity session). After doing this, I rebooted.

LightDM did not load up after this reboot, but stayed at a black screen. I switched to tty1 and logged in via CLI. I removed and reinstalled LightDM, which made it default to Xubuntu desktop as its default session. Another reboot, and I have a LightDM session screen, and upon login, the XFCE power indicator applet is present in the panel, and I can access the power settings from the settings manger.

So it would appear there's now some sort of conflict between gnome-power and xfce-power when they are both installed under LightDM.

Hopefully this is helpful, but this is just my experience.