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..... unplug the HDMI cord while the TV was still on ? I am not saying it did, but it could have shorted out something inside the TV. If it is damaged internally, it might be cheaper to replace than to fix. Hopefully, someone may offer an easy solution. Best wishes.

maybe one of the fixes listed will work for you, have a look at this article >
https://appuals.com/samsung-tv-standby-light-red/

As noted, this is one of the many failures you'll encounter in the TV repair business. To repair you get the service manual, oh wait, I made an assumption you are the usual electronics tech with access to basic gear, safety training and such. But moving on we get the service manual and follow the charts, methods and change boards as indicated. There are some that want to know which board to swap without such work which is not a good idea as that would be guessing and many folk flame on such guesses so I don't guess even if I know it's very likely to be a certain board. Witness how many useless TCON replacements happened and continue to happen.

If you have no compunction to obtain a manual but still want to try, there's YouTube videos such as https://www.youtube.com/channel/UCGfTWLdZHHmhtb-qUb5vb2Q

At six years old I would shop for replacement.