The problem could be that the input on your TV has been inadvertently switched to another input. In other words, the message you are getting is from your TV telling you that your are connected to an input that has nothing connected to it. Select the input menu for your TV and check to see if it is the input that is used to connect to Uverse TV. If you're not connected to the input used to connect to Uverse TV you'll need to switch inputs.
Sounds like the Input was changed on your TV, either with a remote or by accidentally pushing a button on the TV. Find the Input control and cycle through it to get back to your proper input- probably an HDMI input.
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