Yes, this has happened to me also.
732 cold and dark
Turned on Battery switch
Turned on simicon and enabled external power, flicked the external power switch, turned on the power generators when the lights came up. And proceded to fiddle with configuration for a flight plan
after a few minutes noticed I had lost all power yet the ground  power light was still on.
Reset the power generator switch - but it makes no difference.
No response.
Tried the trick of reloading the C172 also, no joy.