This is probably the same issue as bug 355466 and bug 356608 but I file a new one since the way of getting there is differently.

Reproducible: Always

Steps to Reproduce:
1. Connect a second monitor to your laptop (HDMI3 below)
2. Set that display as the Primary
3. Configure you Panel to be on that screen
4. Shut down
5. Disconnect the Primary monitor
6. Power up your machine again

Actual Results:  
The panel is lost, probably still on the primary monitor. It is impossible to use desktop unless you know your tricks (eg. how to pull up KRunner).

Expected Results:  
The Panel should be available on the sole available screen (unless otherwise configured). Especially if there's only one panel.

When the primary is disconnected, no screen is marked as primary anymore:

Screen 0: minimum 8 x 8, current 1600 x 900, maximum 32767 x 32767
LVDS1 connected 1600x900+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   1600x900      60.01*+  40.00  
   1440x900      59.89  
   1368x768      60.00  
   1360x768      59.80    59.96  
   1152x864      60.00  
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.32    56.25  
   864x486       60.00  
   800x450       60.00  
   640x480       59.94  
   720x405       60.00  
   640x360       60.00  
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)