Check that the coaxial cable connections are secure at the modem and at the coax outlet.
Check the Ethernet connections are secure at the modem and at the computers’ Ethernet port.
Power off your modem for at least 10 seconds and then power it back on.
Restart your computer or router connected to the cable modem. This ensures that they receive a correct IP address from the modem.
Check your modem by plugging it in to a different coax outlet.
Check if you have a splitter (a splitter is a small device that has a single coax cable on one side and 2 coax cables on the other side.) between your cable modem and the coax outlet. Remove the splitter and connect the cable modem directly to the coax outlet. If bypassing the splitter fixes the problem, you should probably get a new/better splitter that supports frequencies up to 1000 MHz.
If the problem remains, please contact your Internet Service Provider.