What should you do if your Windows workstation shuts down and restarts spontaneously?

A malicious system will sometimes shut down and/or restart automatically while in use. Although it might be a sign of a hardware problem (a malfunctioning motherboard, for instance), it could also be a sign of a setting misconfiguration or driver problem. Hibernation mode and other sleep settings are more likely to cause issues. If that isn't the problem, a corrupted driver might be. Boot the system into safe mode and check to see whether the issue persists in order to rule out possibilities. If the problem doesn't arise in safe mode, boot normally and start testing what happens as you remove drivers/devices one at a time (sound, video, and so on) until you discover the source of the issue. The boot order in the BIOS/UEFI settings have no bearing on this issue. Even though you might need to enter WinRE to fix the issue, it is not what you should do next. Undoubtedly, you don't yet need to reinstall Windows.