First, make sure your computer or console is actually powered on.
Then, unplug and firmly reconnect both ends of your HDMI or DisplayPort cable. Next, use the monitor's buttons to manually select the correct input source, like HDMI1 or HDMI2.
If that doesn't work, the cable itself might be bad. Try a different HDMI or DP cable. As a last test, connect the monitor to a different computer or laptop to isolate the issue.