When your screen is "Inactive" it shows as red on the dashboard. This means that nodmedia.io can't currently "talk" to your screen. This could be for a few reasons.
1. Is your screen turned on?
2. Is your screen connected to a network?
3. If your screen is turned on and connected to a network is the Nod application currently running on the screen? ie. Does the screen have either the screen's playlist content or a "Welcome to Nod" display showing?
Once a screen comes back online you can look at the Alerts section of the screen to see more information. Go to the Dashboards tab, find your screen within its location and click on it. Then look under the Alerts tab on the right.