If you cannot view the live video or motion detected video, usually the problem is not with the viewer app. You can log in to www.cameraftp.com and click My Cameras to view the camera with a browser. Most likely, the result will be the same as with a mobile viewer app.
The problem is likely caused by the camera. Please check if the camera is still working normally. For IP cameras, you can check it with a web browser, or with the camera manufacturer's app. If you use CameraFTP mobile security camera app for iOS and Android, please check if the app is still running on the smartphone/tablet, and if it has an error message. If you use CameraFTP VSS software, please make sure that the PC is turned on and VSS is running normally.