There is no official fix. Did you try this? I quoted it from the post.

"It did happen every time unless I started the APU before I clicked on any FSX menu items. the APU gen was on.
I tried using the external power and air start when this happens but no joy. I tried the same as Markoz, and the default aircraft with engines running and still no battery power and the engines would not start. I even tried the CTRL E... nothing. Shut down FSX and re-started. No issues everything was working. This happened a couple of times until I put in the electric_always_available=1 line in the config. I know it's not realistic or a fix." Marvic

electric_always_available=1