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Hello, The TV's "Input" was changed which is why the No RGB signal is on the screen. You will need to find the button on the original TV remote, or on the TV itself- labeled "Input" or "TV/Video" to bring the input back to the cable broadcasting.
Now, that is all I can offer without having more details. I would need the following to help further:
1) The model number off of the back of the TV.
2) Is the original remote available?
3) How are broadcasts received on the set- is there a Cable/SAT box connected, or an antenna, or cable directly from the wall to the TV?
Let me know if you need further assistance once you're able to see my reply.