HeroBox returned to the BIOS menu suddenly

Hello. My HeroBox with Windows 10 have worked well for over a year.
But I found the HeroBox returned to the BIOS menu suddenly yesterday.

According to the Windows event viewer’s log, “Unexpected shutdown” was occurred.
First of all, I suspected a power outage. But it wasn’t happening.

Is there any other factor that suddenly returns to the BIOS?