When undervolting GPUs with bios editing you could end up stuck in very bad situations that seem impossible to recover, as the machine crashes before you are able to flash back a previous working image. Let’s go through some steps that can get you out of a very bad situation.

1 The first golden rule is to always have another gpu to boot the system with, while keeping the bricked one inside to get flashed. Best if if it’s by a different manufacturer so you can load the OS without having to load the faulty GPU drivers. Prime example of this is when the CPU has iGpu.

2 Most of the times, the GPU crashes only when the driver loads. Your best bet then would be let windows fail loading twice, going into safe mode, uninstall the GPU from device manager and using DDU to clean the drivers if necessary. Then you will likely be able to flash the previous BIOS version, reboot and install the drivers back.

3 When dealing with notebook, you can usually vbios mod GPUs on MXM modules. On laptops with the GPU on the main PCB, it will be usually embedded in the main system BIOS, wich will be protected from unsigned FW upgrades. It is possible to extract the vbios using UEFItool, program it and inject it back and then flash using external programmer, but that is much harder to accomplish.
On mobile workstation from HP, the MXM module vBios is only read when using macOs or Linux, while windows will use the copy inside the main bios. On DELL workstaton however, the bios is read from the card.

In this situation, you can come across a very interesting situation, where the iGpu is driving the display but the dGpu crashes as you load the driver even when Optimus is enabled, preventing you from flashing back to workin state, as the GPU is completely deactivated when the driver is not loaded. The only solution here is, counterintuitively, switch to dGpu mode (disable hybrid graphic option), uninstall all the drivers in safe mode and then flash as stated in point 2