DIAGNOSIS The screen likely has a buildup of skin oils, dust, and static, or the calibration is slightly off. It's not a hardware failure yet, just needs cleaning and a reset.
FIX First, power it down. Pull the display off the wall plate. This cuts all power. Take a soft, microfiber cloth (the kind for glasses or screens) and lightly dampen a corner with just water—no cleaners, Windex, or alcohol. Gently wipe the entire glass touch surface. Dry it thoroughly with another clean part of the cloth. Wait a minute, then snap it back onto the wall plate. The reboot alone can clear up temporary glitches.
IF THAT DIDN'T WORK There's a hidden touch screen calibration mode. With the display on the wall, press and hold the center of the screen (between the menu icons) for about 10 seconds until you see a service menu. Use the 'Screen Cal' option to re-calibrate. Follow the prompts, which will ask you to tap specific targets on the screen. This resets its touch sensitivity map.
HEADS UP Never use abrasive cloths or chemical cleaners directly on the screen. They can damage the anti-glare coating and make the problem worse. Also, if the screen is only unresponsive in one specific spot, that could indicate a failing digitizer, which would require a warranty replacement.