Display brightness keys suddenly stopped working

[François Colonna]

Hello Xfce team

could you please give a pertinent answer to the recurrent question on
this list :

"how to re-set the brightness plugin for LCD screen" 

Once again, the document mentioned in a previous message :
http://goodies.xfce.org/projects/applications/xfce4-power-manager#lcd_brightness_plugin
is no use : it escapes the question and does not tell how to re-set the
plugin once it has disappeared. It just says "there is a plugin" ! Well
thanks for that scoop !

Hoping to have an answer.
Francois Colonna