Make sure all the wires are secure and everything is plugged into the right sockets. If it's not that, you may have switched to a resolution that your monitor is not capable of displaying. Is it an old monitor or a new one? What resolution are you running it at?
The last resort and the one I'm sure you don't want to hear about is maybe your video cards died.
.. make sure that it's connected to the video card port and not the motherboard port.. silly mistake i know, but i've done it before, or maybe i'm the only one stupid enough to do that.. also make sure that your motherboard is not touching the case and that the risers are secure and everything, the metal against metal will prevent a display from appearing.
This happned to one our computers at school, turned out the cd-rom drive crapped out and it messed up the boot sequence everytime and the monitor would go blank saying no signal. So make sure everything works properly.
my video cards are new, out of the box, 2 nvidia 7950 sli's i doubt they died...if they did ill be pretty pissed, but about the cd-rom...it wont open all the way...this could either be because its messed up or maybe it is the wiring?
there should be two connectors connecting the power supply and motherboard and cd drive together. however, try taking out the cd drive if that's the problem, the bios will still load up regardless of whether or not a cd drive is present.