The answer to your question depends on the context in which "Nothing's on" is said.
At its most literal definition, it means that "Everything is off." More commonly it is used to express the poor quality of the programming choices available (on the TV or the Radio.)
One can imagine a teen aged boy sitting on the couch flipping through channels. He isn't able to find anything that appeals to him, so he utters the phrase "Nothing is on."
If we slide the context back about 80 years, we can picture a similar teen, on a similar couch, who is turning the dial on his radio. He too is unable to find anything that appeals to him, so he says "Nothing is on."
These usages are by far the most common in my experience as a native American English speaker, but the literal definition is used as well. Imagine an electrician is working on the main circuit box of a home. He has attempted to restore power to the home, and he wants to verify that his effort has been a success. He yells upstairs to his assistant, "How does it look up there?"
"Nothing's on!" His assistant hollers in response, and the electrician sighs and returns to his troubleshooting.