Diagnosis
The touchscreen's capacitive sensor has locked up, usually from a software glitch or static discharge.
How to Fix It
Perform a hard reset. Unplug the power cord from the wall outlet. Wait 60 seconds. While it's unplugged, press and hold the power button on the printer for 15 seconds to drain residual charge. Plug it back in and power on. This forces a full reboot of the controller.
If That Didn't Work
The panel might need recalibration. As the printer boots up (HP logo on screen), quickly and firmly press the lower-right corner of the touchscreen three times. If successful, a calibration menu will appear—follow the on-screen dots.
Heads Up
Using screen cleaners or ammonia-based wipes can permanently damage the touch sensor. Only use a dry, soft microfiber cloth.