I had this problem, and looked up online on how to fix it, which is why I stumbled across this thread. I know that this may make me look like a dumba$$, but I realized, after a while of worrying that I would crash, that I was actually out of fuel, thanks to my Virtual Airline miscalculating the required fuel for my trip.
Just make sure that your not out of fuel.
That's just my 2 cents of info. Hope it helps.