# Make sure your computer can actually display the image. If you are using a custom PC and have a discrete GPU, make sure you are plugging it into the GPU instead of the motherboard. If that doesn't work and you have an iGPU (integrated GPU), try plugging the HDMI into the motherboard.
# If none of the above works, check your HDMI pins. Look into the port (on the PC and the display) to make sure that none of the pins are broken. If a port on the PC is broken, try using (if you have) the other ones. Most PCs come with multiple HDMI ports.
# Make sure that everything is plugged in, and test the display with another computer.
# If it still doesn't work, you may need to get your monitor or PC (depending on which is broken) repaired/replaced.
If there is anything I missed in this answer, please let me know!