Just because your power supply says 700, 650, 850 ect ect doesnt mean that it will support the video card, what you need to check is the Amps the 12+Volt rail supports.
Everytime someone has asked in this thread what your power supply was, no one gave a model, brand or anything but what the Wattage was. This is not helpful and people cant even begin to assess the situation.. It could be your Power supply is not strong enough, could be your bios needs a firmware update. Could be the Card is broke.. We dont know unless you provide more than you've stated