The STEM server generates an offline notification to alert you when a camera has not communicated within the past 30 minutes. Offline notifications come through as push notifications (if enabled) and as an Offline Event in the app's right hand drawer Event list.
Offline Events are marked by an icon depicting a camera with a slash through it. The date and time of the event indicates the date and time the camera was determined to be offline. No video data is attached to an offline event.
If your camera remains offline and marked inaccessible, see the article Inaccessible Camera for information on reconnecting it.