Launch Steam from the Desktop icon & click on the Library tab.
Right-click on the FSX entry & select Properties.
![Image](http://www.simviation.com/phpupload/uploads/1504229499.jpg)
From the 'Local Files' tab click the 'Verify Integrity of Game Files' button.
![Image](http://www.simviation.com/phpupload/uploads/1504234467.jpg)
This should check your FSX-SE installation & repair it as necessary. It might take a while