Depending on what was running when you went away, there could have been a system error that caused it to restart and end up in there.
It could also be a drive error, something on the device (virus, malware, etc.) that caused the system to reboot and go straight to BIOS.
If it will exit it, and go back into Windows fine, I would then look into running some good system checks of both software (as well as virus and malware checks) and also run hardware checks.
Now if it won't let you back into the system, and stays in BIOS only, then you need to have the laptop looked at.
Additionally, pets and people messing with a device can cause issues to.