Autopilot fail

Hello, I was making a flight with some direct routes when suddenly my plane began to plummet until it crashed. I had previously configured everything in the FMC and autopilot. What can be?

So, so many issues. And unlikely to be Navigraph related. Usually (not always) it’s User induced error

I understand… and is there a page where it explains in detail how to put the Simbrief and navigraph routes in the FMC? Thank you