Hi! from a hardware perspective, so many things can cause a monitor to not show an image.
-Start with making sure the Monitors are plugged into both the PC, as well as the Power Bar.
-Try a seperate Display cord. (Your VGA, DVI, HDMI, or DP Cables could be failing)
-Try a different port. (most recent motherboards support both HDMI, as well as DVI,) or if you have a video card installed, be sure to try your on board port.
-Try a new monitor if it does not work
-if this does not function, try putting your windows Disc into the Drive, and see if the PC Will boot to this
-If this does not fix it, then look into disassembling the device, vacuuming out the device and components with an Antistatic bristle head, adding new thermal paste, and reassemble the device.
-If this does not work, try a new Video card. if this doesnt work, likely your motherboard if failing.