FS does not load 747-400 default flight

I always had set as start default flight: PMDG 747-400 United at KMIA cold and dark.Existing problem now when I start FS I get this message:"The requested flight file is corrupt or can't be found. Flight Simulator will now try to load the default flight."When I press OK, the original CESSNA default comes up.Need help, thank you!Pepe