Check your C:\ drive:
^ This guy was stuck on the first loading screen and wouldn’t load any further. He later found out the cause was he ran out of space on C:\. You might have installed the simulator to a different drive but it still might be writing files to C:\ in these locations:
C:\Program Files (x86)\Steam\steamapps\common\MicrosoftFlightSimulator
C:\Program Files\WindowsApps\Microsoft.FlightSimulator_[Your Version Number]_x64__8wekyb3d8bbwe
C:\Users\[Your User Name]\AppData\Local\Microsoft\WindowsApps\Microsoft.FlightSimulator_8wekyb3d8bbwe
C:\Users\[Your User Name]\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe
C:\Users\[Your User Name]\AppData\Local\Packages\Microsoft.FlightDashboard_8wekyb3d8bbwe
C:\Users\[Your User Name]\AppData\Roaming\Microsoft Flight Simulator
C:\WindowsApps\Microsoft.FlightSimulator_[Your Version Number]_x64__8wekyb3d8bbwe
C:\WpSystem\S-1-5-3876622tet-771027889-3555349395-1001\Appdata\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe
Important: Windows 10 by default hides the “AppData” folder, so you will have to go to “View” in the menu of File Explorer, and select “Hidden items” so as to see it.