Assuming a version of MS Windows, look in the Device Manager for Network Adapters, wherein your Realtek card will have a listing. Right-click for Properties and see if Device Status states “This device is working properly.”

Also, with the system turned off and unplugged, pull the card out, clean the contacts with a soft eraser, and put it back in. (This also re-seats the card, which is something to do anyway for intermittent operation.) Test again.

Finally, the cards are cheap, so get another one. The first one probably was zapped by static electricity or a surge-spike (or manufacturing weakness by Chinese slave labor on a bad day). If it ever happens again, put all your cabling through a surge-suppressor, which may it’s a good idea regardless (or pray for all the poor communist slaves the world over).