One problem solving approach is to uninstall the NIC driver (be *certain* you have the existing or new driver to re-install). Shut down. Restart and install the existing/new driver.
If it installs and works, there was a driver problem.
If it won't install/won't work, you likely have a NIC problem or a NIC connector problem.
You can also try the PC utilities that come with your machine (e.g. PC Doctor on some machines).
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