Please try to reboot the system to see if it can bring them back. And please go to the NVR's camera management page, tap on the little yellow triangle regarding the problematic camera, and see what's been indicated up there of the IPC.
The camera may get offline if the system password or the network is changed. Some resolution and recording settings could also cause the system to drop cameras offline.
If needed, we can try to reset the camera to restart. Here is a video about how to do the hard reset on the camera. You can refer to this video to check if your camera has a reset button and how you do the camera reset.
How to reset the IP cameras: https://www.youtube.com/watch?v=8n86WRFqwv0
If it doesn't work for you, please try to follow the attached PDF instructions to use the IP Portal software to reset the camera. Otherwise, get in touch with the LaView support team for more help.