Take the battery compartment cover off. Roll the batteries, in place, with your fingers.
Does this start it working again?
If not, press the black button you can see there and hold it down for just over ten seconds.
Does that bring it back to life? (Even if you have to re-pair it).
If still no go, get your phone, point its camera at the little window on the top of the remote, and press the volume controls or the On/Standby button on the remote. Can you see, on your phone screen, a light in the window when you do this?
If not, try new batteries.
NB: Note that all other buttons on the remote, the ones that talk to your NowTV rather than the TV, won’t show a light. This is normal; it has to be one of just those three for this test.
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