This seems like the most active/longest thread related to this issue and I'll post a quick guide of how I fixed my EVGA GTX 670 4gb after trying to change it's hardware ID to match a Strix 970 in order to enable differentSLIauto. NvFlash was no longer able to recognize the adapter in DOS, Linux, or Windows after the hardware ID change, and I can't recommend against it more strongly for the purpose I intended: Windows seems to assign a driver profile based on the cards hardware ID, and although it was showing up in hardware manager (often with a "bad" or yellow icon driver after reboot, easily remedied yet still..) I couldn't see the card in the nVidia software or nvflash as previously stated. Anyways you're looking for a fix.
This will only work with nvidia cards as far as I know:
1) Disassemble the card until all major IC's are visible. Google each part number till you find a memory chip with a CS# option on Pin 1 (usually) and ground on pin 4 (usually).
2) Once identified, reinstall the card allowing access to BIOS chip you found from the previous step. Ideally, the heatsink for the card should be installed and the fan running, other components should survive.
3) Flash the BIOS manually. By this I literally mean use a pair of stainless steel tweezers to short out the connection between pin 1 and 4 (unless otherwise identified, CS# = pin 1 and GND/VSS = pin 4) simultaneously WHILE windows is running. Yup. If it works you'll hear the found new hardware bada-boop. You'll be fine, AS LONG AS YOU: don't touch any other components, discharge static beforehand. I'm seriously not responsible for you accidentally tasing yourself. Continue to step 4 if inconclusive.
3a. If successful, DO NOT TURN OFF THE PC. Start NvFlash and reinstall the original BIOS (why wouldn't you make a backup..?) or a known working BIOS. the card should be fixed.
-NOTE: Often times you'll hear the CS# pin referred to as the STRAP_SUB_VENDOR pin, ignore this nomenclature and use what I provided in step 1 (and step 3 if GND is not easily recognized), it will not help you.
4) If not successful this card is well and truly dead, but you can sell it to someone who thinks they're smarter than you on ebay for a bit. I've seen cards resurrect from replaced capacitors and other discrete SMT components but unless that's trivial work to you make better use of your time.