One of my HDMI inputs isn't working?

Check that the panel is switched onto the desired port, ensure you aren't using an adapter or dock. 85% of display issues are caused by faulty adapters or docks that can't handle 4K resolution. Try using a different HDMI cable, try using a different laptop (one that you know works).

Change your display settings on the device you're trying to connect (lower or higher resolution). Right-click on your computer desktop and select display settings. 

Once display settings open scroll down to display resolution and filter through the different display resolution options until the image fills the entire screen and the touch works correctly. 

Change your display setting on the device you are trying to connect. Press the 'windows' key and the 'P' key on your computer keyboard to bring up the display settings. Select to 'extend' rather than 'duplicate'. 

Try using a different input port on the panel to double-check that the port on the computer isn't faulty.