did you try to re-connect Power PCI-E cables to the GPU? I cant imagine that you can flash them if they are not connected to the mainboard. If you did and windows didnt find your card…try to run hiveOS and check if Linux finds it.
I think you flashed a wrong bios file…you didnt told us what file you flashed or what you changed…I think that could be the problem…I already flashed some stuff that killed my 5700 too…it´s very easy to kill a 5700 with flashing the wrong stuff