Defective cable or port most likely. If replacing the cable doesn’t work, try checking the driver version for the network interface on your PC in device manager. Check the advanced settings. Try forcing 1 gb mode there. If that fails, try rolling back the driver. Sometimes windows will update those drivers alongside other windows updates. If you’re still SOL but other devices are fine, it’s most likely a defective network card. Disable it and get a cheap replacement. A PCI express replacement is preferred, but USB ones have come to perform really well, in general. But you still have more bandwidth available and lower latency directly on the PCI-E bus. But USB is an option if you don’t have any spare slots.