First of all, do you get an error message or motherboard beeps, or it simply won't boot. Something similar happened to me yesterday, when I unplugged and plugged back psu connectors. I reseated the ram and booted.
Second, as dumb as it may sound, did you plugged both the power and the data cable? (I say that because it happened to myself once).
Third, are both the HDMI and DVI ports located on the graphics card/motherboard? If the DVI is on the graphics card and HDMI on motherboard and you uninstalled the integrated graphics driver, then that may be the cause.