A few reasons a recorder can go offline:

power surge, blackout, new ISP, ethernet cable came loose, PPPoE was enabled.

If there is a computer on site, ping the recorders IP address to see if the recorder is communicating with router.

Start by checking the wire. Make sure it is plugged in and is working. Try swapping it out to be sure. 

If the wire is loose or unplugged, Network status will show Offline.

Go to TCP/IP under Network and check to see if DHCP is enabled. If not, enable and give it a moment for the router to assign the recorder a new IP address. 

If DHCP is already enabled, disable and click apply. refresh the page. Enable DHCP again and click apply. Reboot the system.

Titanium (Obtain an IP address automatically) 

If TCP/IP is grayed out, go to PPPoE and disable it. Once PPPoE is disabled, go back to TC/IP and enable DHCP and click apply. Reboot.