While attempting to troubleshoot an upgrade of AMD CPU's in an R815 server running Windows 2012 Server, I set the boot mode from UEFI to BIOS mode. (In retrospect, it looks like this is a neophyte mistake!) The machine will no longer POST, and immediately shuts down. I found the following in one of the Dell manuals:
"Troubleshooting System Startup Failure
If your system halts during startup prior to video output or LCD messaging, especially after installing an operating system or reconfiguring your system's hardware, check for the following conditions:
• If you boot the system to the BIOS boot mode after installing an operating system from the UEFI Boot Manager, the system will hang. The reverse is also true. You must boot to the same boot mode in which you installed the operating system. See Using the System Setup Program and UEFI Boot Manager."
and...
"Choosing the System Boot Mode
The System Setup program also enables you to specify the boot mode for installing your operating system:
• BIOS boot mode (the default) is the standard BIOS-level boot interface.
• UEFI boot mode is an enhanced 64-bit boot interface based on Unified Extensible Firmware Interface (UEFI) specifications that overlays the system BIOS. See Entering the UEFI Boot Manager for more information on this interface.
You must select the boot mode in the Boot Mode field of the "Boot Settings Screen" of the System Setup program. After specifying the boot mode, proceed to install your operating system in that mode. Thereafter, boot the system to the same boot mode (BIOS or UEFI) to access the installed operating system. Trying to boot the operating system from another boot mode causes the system to halt immediately at startup."
However - I haven't found any information about rescuing the situation, so that I can get back to any kind of booting process. (Have tried resetting NVRAM, and booting from USB with DOS-formatted stick with BIOS replacement - without success).
Any pointers would be gratefully accepted!