As you probably have searched already I'm just replying to say it's happened to me a few times over the years. With Windows but not FSX. I seem to remember most suggested solutions online did not work and I had to drop in into a specialist PC repair guy. He fixed it quickly abd the charge was almost insignificant.
But I do remember a couple of times fixing it myself by following online suggestions by making a bootable drive and starting windows from that . https://www.youtube.com/watch?v=iGGNW6gxTm8
Simplicity is the ultimate sophistication. – Leonardo da Vinci
-Open Steam -Go to Library (at the top) -Right click on the Game -Select Properties -Click on the Local Files tab -Select Verify the Integrity of the Game Files -Steam will verify your local files with their database and download anything that is missing/corupted
Very occasionally this happens when one or more of your RAM sticks is a little bad. It still works 90 to 99% of the time but randomly doesn't work and forces Windows to restart. This happens even more so if you get the blue screen across various programs and not just FSX. Anyway, as long as FSX continues to work fine and doesn't get the blue screen, great.