What does the flashing file folder and question mark mean?

My computer is crashing in the middle of the night about once per week, and when I wake up it shows the flashing file folder with question mark. I don’t see any info on this except as something that shows up when you try to restart your computer. But this is showing up when I’m asleep, NOT restarting the computer. As a result, I have to restart my computer, which goes fine, but why does this happen in the first place? It is doing it both with the current OS, and the prior one. Thank you!