Normally i would suggest that its a GPU problem but since you said you replaced it and still nothing happened we'll move on.

Do you hear any beeps after turning it on? If you do go here and check out what they mean: BEEP CODES

Another possibility may be that your 4-pin CPU connector isn't connected. Check that as well.

Lastly, if all fails reset your CMOS. Unplug your PC. Gently pull out the circular silver coated CMOS battery, traditionally on the bottom right side of your Motherboard, and wait for 10 seconds. Put in back in and try booting up again.