Why has my TV remote stopped working?
Work through the causes cheapest first — the majority of "broken" remotes are not broken.
Batteries, including ones that look fine. Infrared remotes draw very little current, so batteries degrade slowly and the remote works intermittently for a long time before failing. Replace both with fresh ones of the same type rather than testing, and do not mix old and new. Also check for corrosion on the contacts — a white or green deposit from a leaked cell prevents contact and can be cleaned with a cotton bud and a little vinegar or isopropyl alcohol, then dried.
Test whether it is transmitting. This is the single most useful diagnostic and takes ten seconds: point the remote at a phone camera and press a button. Most phone cameras detect infrared, so you will see the LED flash on screen even though it is invisible to the eye. If it flashes, the remote works and the problem is at the television.
Note that many newer phone cameras filter infrared more aggressively, and the front-facing camera often works better than the rear.
Line of sight and obstruction. Infrared needs a clear path. A soundbar, a plant, a decorative panel or a new piece of furniture in front of the TV's receiver will block it.
Interference. Strong sunlight, some energy-saving and LED lighting, and plasma displays can swamp the receiver. Try in different light.
Stuck buttons. A button held down continuously — often from debris under the rubber or from sitting on the remote — blocks all other commands. Press each button firmly and listen for anything that does not click back.
The television itself. Try the TV's own buttons. If those work and the remote transmits, the TV's IR receiver may be the fault.
Bluetooth and RF remotes are different: they need re-pairing, usually by holding two buttons together — check the manual.
Power cycle the TV by unplugging it for a minute, which resolves a surprising number of cases.