I have to agree with @mixxy1010 here; your LCD is damaged and needs to be replaced. Fortunately, Chromebooks tend to make it fairly easy to replace a broken display panel.
I couldn't find any specific information on replacing the screen on your particular model, but it appears that HP's line of Chromebook 15 models are all pretty similar in how the screen comes off. In most cases you just pry off the bezel around the screen, then there are four screws holding the LCD in place. Remove those and flip the LCD down and you'll find a single connector on the back that is usually secured with tape. Just peel off the tape and you should be able to unplug the LCD.
Here's an example of a guide to replace the LCD on a different Hp Chromebook 15 model; hopefully yours is similar enough that it will work for you.
HP 15-bs168cl Screen Replacement - iFixit Repair Guide
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