Do you mean it's not launching, and giving an error message when it fails?
If that's what you mean, try going to your FSX.CFG file. In WIN7 Pro it's located in C:\Users\<Your USER NAME>\AppData\Roaming\Microsoft\FSX .
Make a back up copy of it before you do anything else!
Open it with NotePad, included with Windows. ONLY ever open it with NotePad, nothing else. Never ever never never!
Scan down through it and located the heading
[USERINTERFACE]
Under that, locate the line SITUATION= . What follows the equals sign ( = ) doesn't matter. Why? Because you need to delete what's there!
Save the file off, and see if that fixed the problem.
Another possibility is that your Logbook.bin, in the same location, has gotten corrupted. Try renaming it, with FSX NOT running. Anything you like. Logbook_BAK.bin, Logbook.NIB, whatever. Then see if FSX will start. If it will, download a logbook repair program. There are a number of them out there. Google them and you'll find one to your liking.
If those two solutions don't solve you problem, we will need more information, like the report from the Windows Error Reporting system. Things like that.
Hope this helps!
Pat☺