Camera was online and now it's not
Check when the camera went offline in the Health tab.

If you have more than 1 camera at the same site - check if other cameras are online and if they went offline temporarily during the same time.
Direct IP camera
Router changed it’s IP address - all cameras from the same location should be down.
Update the IP address for the camera in the Settings->Cameras->Connection tab.
Use DDNS (like no-ip.com) instead of IP address.
Router may reset it’s port forwarding table, this happens often after router’s f/w upgrade or reset.
Configure port forwarding again, using the same ports.
Uplink Camera
Camera can’t reach DNS server
Configure an extra DNS server on the camera, it could be for example 8.8.8.8
Uplink plug-in stopped working (unlikely)
Log-in to the camera in the local network and check that the plug-in is installed and running.
Download logs from the camera
Reboot the camera