There are a few causes to this issue but most of them are related to network connectivity. This means you will need to check the firewall on your computer and then make sure the Internet is working fine. Additionally, the cameras need to be configured on the DVR to accept remote connections. If this feature is turned off, then you will have problem accessing them.
Another possibility is related to the port forwarding. If your cameras are online, but "behind" a router, you might need to open ports on the router for the DVR to be able to connect to the live streaming.