I’m not a professional tech, but it sounds like screen is damaged internally. If you want to fix it yourself, look here for the tools and price of a replacement:
OnePlus 6 Repair
Here is the link to the screen on the IFixIt Store, and to another seller online:
OnePlus 6 Screen
Here is a guide I found online also:
Otherwise, you should probably take it to a shop. Not sure on the price there but I bet it will be a good bit more expensive than doing it through IFixIt.
And if you want a good kit with all of the tools you need for this and future repairs except for a heat gun, check this kit out here: Pro Tech Toolkit