@SJGREENWOOD
Ordinarily, when you have a screen issue like this, you connect an external monitor to determine if it is the screen or the video chip. But since you can't see anything on the monitor, there is no real way to tell that way.
You should try running diagnostics to see what they find. Press the Esc key repeatedly while rebooting and when the HP Startup Menu appears, press F2 to run diagnostics.
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