You will need to "tell" the TV which tuner to use - the old analog signals or the digital transmissions. Switch over to Digital has just completed - so there are no longer any analog signals.
Somewhere on your TV or remote will be a control for Input or Source - to tell the set which signals to process.
If used as a games monitor it will have been set up to process signals from an external source - the games box.
Peccavi
August 2013