As discussed many times on the forum, GSX doesn't act in any way on your airplane. It only checks things like the doors status or the engine status, but it doesn't try to do any changes to any system.
In your case, it seems that, after shutting down the engines in order to be serviced, your airplane didn't had a source of power for the instruments and to restart the engine, most likely because the battery has been exhausted.
What you need to do, depends on the airplane you use and it's entirely unrelated to GSX. If your airplane simulates the availability of a ground power supply, you can use that. Or, if it has an air start unit, you can use that too. Or, if it simulates an APU, you can leave it on while being serviced.
On a default airplane, or a 3rd party airplane that doesn't have any means to supply power with engines off, you can use FSUIPC, which has an option to prevent battery drainage, which is way too fast by default.
