Normally that will happen when systems do not have electrical power. Verify that power is on for all the avionics systems. It is possible that there is not an avionics switch (haven't flown for a while so don't remember) in which case you will have to turn on the avionics using another aircraft then switch back into the B52. Hope this helps

Dave

As above.
I have not been able to find an avionics master switch either in the manual or the cockpit.
I mapped the master avionics on/off FSX command to key "R" for now and this switches on all kinds of things, including but not only, the autopilot and radios.

I expect an expert will come along in due course and enlighten us.

Regards,
Nick

I don't know if you know how to do this, in case you don't, here is how.
Carenado installations put their gauges into the default gauges folder, so
all you need to do is edit the B 52 panel.cfg file.

First, select this entry from your King Air panel.cfg file
[Window Titles]
WindowXX=GPS,
XX being the number assigned in your aircraft's panel.cfg.
and copy it into the same section of the B52 panel.cfg file, under the
Window01=Simicons entry and alter it to Window02=GPS.

Next, scroll down to the [WindowXX] entry in your King Air panel.cfg file and select the whole entry, including the title.
Copy it into the B52 panel.cfg file under the [Window01] entry, just above the [VCockpit01] entry.
Alter the [WindowXX] entry to [Window02]

Save the panel.cfg file and the next time you fly, pressing SHIFT+3 will open and close the new 2D window.

Caveat, I don't have the exact Carenado model that you do but this guide would work for the two aircraft I do have.

Regards,
Nick