When you say TV channels do you mean over the air Freeview channels via a TV aerial.
If so it sounds like you have the NOW Freeview Smart box with the blue NOW logo on the top casing.
What I would try first is step 6 in this linked article below.
If that doesn't help then try a full factory reset by flipping the NOW Box over and hold in the small reset button for around 20 seconds.
Please note logging into the My Account on the NOW Box has changed since the latest software update and there is no longer a My Account tile box on the bottom grid row of the NOW home screen, where instead you need to access My Account by clicking on the gear wheel symbol on the top of the NOW home screen.