Error code 651 in Windows 8 PC

Meaning that the modem is not communicating with your ISP? If the modem is owned by the ISP I'd start with a call to them.

If it is your modem, are you using it bridged (modem only) or as a router also? Either way, unplug the 5100 and shut down the computer. Connect computer to modem via ethernet. Plug in modem. Wait 20 to 30 seconds (observing any LEDs on the modem/router of course) and then boot computer. Any difference?