Just one device or multiple devices on your network?
Do you have access to the router logs?
First check all physical wired connections starting with the connection from your ISP. Remember to check power connections as well from outlet to device.
Make sure that all connections are snug and secure. No bare metal showing, no kinks, no rodent/pet chews, no corrosion. No visible physical damage of any sort.
Then swap out existing ethernet cables one by one with a known working ethernet cable.
Are you using an internal ethernet adapter, built-in motherboard adapter, or USB ethernet adapter? Make sure any removable adapters are properly seated and connected.
Then call your ISP. Have them run some tests from their end. May take a...