First, Verify if the monitor is powered by the AC/DC wall plug, then verify the HDMI/DVI/VGA/Display Port cables. If not working, go to the troubleshooting step #1.
TROUBLESHOOTING:
1: Look if the PSU is powed on and the power cable is not damaged
If this doesnt work go to step 2
2: Open your case
3: Locate the GPU (Grphics Card)
4: Verify if the GPU power cable is correctly installed ( the 6 pin/ 8pin connector on the top of the GPU)
If this doesn't work go to step 5
5: Verify if the GPU is correctly seated in the PCIe slot and cliped
If this doesn't work go to step 6
6: Remove the CPU cooler
7: Gently remove the CPU ( If you dont know how to do it watch this video: https://www.youtube.com/watch?v=tVLUIRF7K4U)
8: Replace the CPU
9: Replace the cooler
That should be working now, if not, the GPU of the motherboard is broken