Lost my panel logging out

[AG]

Hi list

On Debian Squeeze, running Xfce4 4.6.1, after I logged out of one 
session and came back in later there was no panel.  This is the main 
panel with the default system tray and launch icons, clock, etc.  I can 
get the panel running by calling

xfce4-panel &

but I want a more satisfcatory solution than this short-cut expediency.  
Can someone please give me a steer for where I am supposed to track down 
this option, because the Settings Manager's panel menu item doesn't 
respond to mouse clicks, and I am unfamiliar with Xfce4's config files 
and don't want to mess something up.

Thanks for any pointers where to look/ what to look for.

Cheers

AG