Try just powering off the monitor and then powering up again.
Try unplugging the monitor from the wall and plugging it in again.
Try removing the data cable from the monitor. Check there are no bent pins and re-seat it. Do this with both ends.
Try replacing the monitor with a different one.
If any of the above result in an image on the monitor it is likely to be a monitor fault.
If none of these work it might be a graphics card issue. Try reseating your graphics card(s).
If you have more then one graphics card try reconfiguring your system so only one is used. Does this work?
If you can replace the graphics cards with a known good one try taht and see if that works.