This problem seldom happens, but if it does, please try the following:
1. Reboot the cameras by unplugging and reconnecting the power cable, and see if it resolves the problem. If it does, please check your camera firmware version.
2. Please make sure your network is using DHCP, and not static IP address. If assigned a static IP address, a camera may not be able to re-connect to your router properly.
3. Reboot your router, and see if cameras goes online. Check your router setting, and look for the ip address range of your router. If it is 192.168.0.X, (which is usually 192.168.0.1), then please change it to 192.168.1.1, please note the only difference of this changes is '0'->'1'. You may need to reboot other online devices to regain ip address after this configuration. To find router ip address, refer this link for reference, The settings in other routers will be different, but it's usually not too difficult to find the IP address range.