You tried it in multiple computers and it doesn't work, seems like a dead graphics card.
At that point you could try to fix it using several methods, what do you have to lose right?
Check HDMI cables? check power supply cases? does the motherboards have a debug LED with some form of post code numbers.
Install 2 GPUs, or boot off of the integrated graphics on a CPU, once in Windows see if the card is recognized, you could try to flash/re-flash the vBIOS, validate the BIOS in GPU-Z, on you could use techpowerup to find your BIOS to flash with ATiFLASH, if it works congratulations, if not then take off the shroud + cooler, check for physical damage or burning spots, if you find one then you know where the problem is.