DIAGNOSIS Your TV isn't detecting your HDMI devices. The most common cause is a mismatch in the TV's input signal format.
FIX Force the TV to recognize the signal. Grab the Samsung remote. With the problematic HDMI source on, go to Settings > General > External Device Manager > Input Signal Plus. Turn ON the specific HDMI port your device is using (e.g., HDMI 1). This enables the full bandwidth for modern consoles and players.
IF THAT DIDN'T WORK The HDMI cable might be faulty or not high-speed enough. Swap it with a known-good cable rated for HDMI 2.0 or higher. Also, power cycle the TV and the external device: unplug both from the wall for 60 seconds.
HEADS UP Some soundbars or receivers can interfere with the handshake. Try connecting your device (like a game console) directly to the TV's HDMI port first to isolate the problem.