There are a few scenarios which can cause this
1) There is no application scheduled to the display
To verify this, please note the "Display ID" as highlighted below
Then, login to the cloud console and navigate to your device and open the "DISPLAYS" tab. You will have to find the display with the corresponding Display ID
Once done, please double check below this if any content is scheduled
Please note: If a new display is plugged into the device, it may get a new display ID and will need content scheduled onto this.
2) There is no application scheduled for the current time
For example, you may have your display schedule setup to show content in the morning at 7am - 12pm, but the current time is outside of these hours.
3) There is a possible cabling/hardware failure
Please refer to this article to qualify if this is the issue