When NCS stops to show a video stream or it is frozen it usually depends on two things:
lost connection to camera via LAN or wifi. NCS try to reconnect and when LAN or wifi is back again the streaming continuous. Look in the Event Viewer or logfile where the re-connection is listed.
corrupt data stream. When the camera is connected to NCS with the rtsp protocol to get video and audio the signal is compressed (encoded) with the h.264 (or h.265) algorithm. In NCS this data stream is decoded to video and audio and is shown as a streaming video. When that data stream gets corrupt for some reason the decoder cannot work anymore and the video stops. To get this to work again the encoding/decoding must be reset by disconnect/connect the camera or reboot the camera. To solve this test to switch data stream between rtsp_,tcp or rtsp_,udp. Upgrade firmware in camera and if running on wifi increasing signal strength usually helps.
If audio is not necessary I recommend to switch to use the mjpeg data stream if the camera support that. Mjpeg is much less sensitive to data errors. It use a little bit more bandwidth, but that is usually not a problem.