What are your full hardware specifications?
Try disconnecting the power cable from the back of the PSU (Or flipping the switch to the "0" position on the back of the PSU) then removing the graphics card completely (Or installing a different graphics card entirely), THEN removing the CMOS battery for five minutes. During that five minutes press the power button on the case continuously for 30 seconds to deplete any residual charge that might remain. After 30 seconds you can stop pressing the power button and wait for the remainder of the five minutes then reinstall the CMOS battery being sure to put it back in with the correct side facing up. Reconnect the power cable to the PSU or flip the switch back to the "I" position. Then power on the system and let it do whatever it wants to do for a minute or two. If it cycles, so be it. If it powers on and shuts down immediately, so be it. After a couple of minutes, power it off, flip the PSU switch off again or disconnect the cable and reinstall your graphics card.
Once the card is reinstalled, plug the cable back on and see if the system will power up and has a display signal.
If you have another graphics card you can probably avoid all that by simply powering off, unplugging the PSU from the wall, installing the different graphics card, plug back in and power on. Often the change in card will trigger the display to begin working again and then you can just reverse the procedure to go back to your original card. No idea why this happens on some systems, but it does. Can also try just using a different TYPE of display output, as in, switch to HDMI from DP or whatever.
Out of curiosity and because it might matter, WHAT setting in the BIOS did you change?