The first step is to review the event viewer, I’m not good at that, but there are people here who are.
If you’re overclocking RAM, well, the first thing I’d do is stop overclocking and see if that helps. Doesn’t matter if it’s been working fine for months. Most programs aren’t as ram intensive as MSFS. If you have a bad stick in there, it may only get used by MSFS. If that doesn’t help, I’d try replacing the ram, or, if you have 32GB, try just using two sticks, and then swap for the other two sticks and see if it starts. IOW, figure out a way to take the RAM out of the equation. You could easily have damaged it.
I’d also look into your MS login. I had a problem like this way back when there was a glitch in the update download, and I had to go through and remove the MS Store app, the Xbox app, load another Xbox app, (rebooting all the while), and some other stuff to finally get the login working.
You could have an SSD that’s going bad, too.
In any event (pun intended), the first step is to review the event viewer and see what’s going on at boot.