I'm sharing this so it may help anybody else & save them the grief I went through.
We suffered a power cut/power surge (the Grid confirmed it was more likely the latter) at 3am one morning recently & when we tried to use our Samsung 4K UHD Curved TV we found the following stopped working:
1. Remote control would not respond to powering on the TV, only a hard power cycle from mains allowed us to turn the TV on and use the remote control
2. Bluetooth stopped working - our Soundbar could no longer be paired as it didn't find any devices under device list. Troubleshooting with Samsung phone support showed the Soundbar was visible to other devices such as my smart phone & would pair no problem but not to the TV. The TV's Bluetooth MAC address (which can be checked under system settings) kept disappearing
3. Following on from the above issue number 2, I could no longer mirror content from my smart phone such as playing music or youtube videos to my TV, it just showed a spinning wheel icon constantly & froze
4. None of the built-in apps such as Netflix would load any more
The TV speaker still worked as did playing DVD's from my DVD player & cable TV from Virgin Media!
Unfortunately the 2 year warranty recently expired but I called Samsung anyway.
Samsung telephone support first had me reset the remote control & change the batteries to no avail. They then had me reset the Soundbar but again to no avail. Finally, they asked me to reset my TV under settings and set it up again but half way through it froze and became completely unresponsive.
I took it to a Samsung repair centre & they advised after having it for 2 weeks that nothing was wrong with it as they tested all the issues listed above (1 - 4). This was 2 x 80 miles round trips in COVID times and stuffing the TV (just about) in my car as Samsung don't do pick-ups or come to you.
Happily I brought the TV home & re-plugged everything back in (HDMI cables, network cable, USB stick). Again, I had the same issue!
Recently I had installed LAN powerline adapters in my house due to WiFi drops & work laptop/VPN issues so I turned all that off to test but it made no difference.
Being close to replacing the TV & giving up I noticed the USB stick inserted was not appearing under the Source list as I wanted to copy some yoga videos I had downloaded on there as I could no longer use mirroring from my smart phone or the TV's youtube app. I inserted the USB stick into my PC & it moaned saying I had to re-format it to use it. Chkdsk failed with a generic error message. So I used a data recovery software EaseUS to recover the data to my PC first, re-formatted it & copied the data back on it.
I plugged the USB stick back into my TV & then in the morning my wife started going crazy saying everything was now working again! Sure enough I just re-tested all the issues (1 -4 above) to confirm I can indeed use the remote control without having to turn the TV on/off from the mains socket, paired my Soundbar via Bluetooth, checked & loaded built-in apps such as Netflix and mirrored content from my smart phone to the TV!
My conclusion is that the power cut/surge corrupted the USB stick somehow and while it remained inserted made all the above issues arise! Yes I should have removed the USB stick in the first place then test but you just don't think of these things with unusual symptoms described above.