Take the battery out and reset the BIos on your MOBO. Turn On the computer without any GPU, and set the wanted settings in the bios. Shut down the computer, put all the cards in and turn it ON with the screen attached into your motherboard. If there will be a problem, you will get an option to restart the computer or go to "advance options". Choose the restart and your computer should turn on

I had the same problem when i wanted to put some new cards on the motherboard..