Things to check:

Boot devices in BIOS. I had a system do this to me and I found someone had disabled the hard drive as a boot device. Actually, they enabled HDD-1 as the boot device. I found HDD-0 nowhere near the HDD list in the BIOS, hence where the confusion came from.

PC Health monitoring in BIOS. Disable this in the BIOS in case someone deleted the driver for it or in my case removed an unknown device from the hardware devices list. If the PC Health monitoring drivers are messed with, the system will not boot until it is disabled in the BIOS. The OS thinks that because it cannot monitor the system health any longer there must be a problem and it shuts down to save the processor, etc.

Good Luck!
Stuart