If you haven't, please try an alternate monitor to understand if this is either a hardware or a software failure,
if the issue does not appear on the alternate monitor, you may need to replace the display assembly on your computer,
if the issue appears on the alternate monitor/TV as well, re-install the graphics card drivers & update it from the official HP Website: Click here
If you don’t have an alternate monitor, I recommend checking if the issue appears on the BIOS screen (restart the computer and tap on F10 to access BIOS)
If the issue does not appear in BIOS, reinstall the graphics card drivers & update BIOS using the above link,
If it does, you may need to replace the LCD/Repair the computer:
If the issue occurred after an accident (such as dropping it on the floor, liquid spilled on it, etc.) there could be a hardware failure and in such cases, you may need to get the computer repaired via HP (there could be a charge involved if the PC isn't covered by the accidental damage protection plan)
Reply with the result of the above-mentioned steps, for further assistance.
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