You could try to factory reset the device to see if it’s a software issue. If that doesn’t do anything, then my best guess if that your LCD back light has gone bad and that you may have to replace the entire LCD assembly.
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You can purchase a replacement LCD here for around £30: https://www.aliexpress.com/item/For-Amazon-Kindle-Fire-HD8-HD-8-Replacement-LCD-Display-Touch-Screen-Digitizer-Glass-Assembly-8/32763080973.html?spm=2114.search0104.3.8.183f254dawgqjP&ws_ab_test=searchweb0_0,searchweb201602_1_5724111_10065_10068_10130_10547_5724211_10548_5890011_5724311_10696_10192_5724011_10190_10084_10083_10618_5970011_10307_10820_10301_10821_10303_5910011_10059_100031_10103_5725011_5990011_5724911_5980011,searchweb201603_55,ppcSwitch_5&algo_expid=2ab55feb-2a38-469f-8eac-b023c93ed25e-1&algo_pvid=2ab55feb-2a38-469f-8eac-b023c93ed25e&priceBeautifyAB=0
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You can purchase a replacement LCD here for around £120: https://www.aliexpress.com/item/Original-6-inches-HD-Eink-Screen-for-kindle-oasis-2016-eBook-Reader-LCD-Display/32867192348.html?spm=2114.search0104.3.1.e790765e5b1qg2&ws_ab_test=searchweb0_0,searchweb201602_1_5724111_10065_10068_10130_10547_5724211_10548_5890011_5724311_10696_10192_5724011_10190_10084_10083_10618_5970011_10307_10820_10301_10821_10303_5910011_10059_100031_10103_5725011_5990011_5724911_5980011,searchweb201603_55,ppcSwitch_5&algo_expid=22493d07-9be9-4538-bd11-d15047a6f119-0&algo_pvid=22493d07-9be9-4538-bd11-d15047a6f119&priceBeautifyAB=0
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However you will be expected to wait up to 40 days.
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The only other option could be, take it apart, remove the screen and put it back and ensure that the connector on the ribbon cable is clean and seated correctly. The cable may have become positioned wrong from being knocked or dropped.
You could try to factory reset the device to see if it’s a software issue. If that doesn’t do anything, then my best guess if that your LCD back light has gone bad and that you may have to replace the entire LCD assembly.
You can purchase a replacement LCD here for around £30: https://www.aliexpress.com/item/For-Amazon-Kindle-Fire-HD8-HD-8-Replacement-LCD-Display-Touch-Screen-Digitizer-Glass-Assembly-8/32763080973.html?spm=2114.search0104.3.8.183f254dawgqjP&ws_ab_test=searchweb0_0,searchweb201602_1_5724111_10065_10068_10130_10547_5724211_10548_5890011_5724311_10696_10192_5724011_10190_10084_10083_10618_5970011_10307_10820_10301_10821_10303_5910011_10059_100031_10103_5725011_5990011_5724911_5980011,searchweb201603_55,ppcSwitch_5&algo_expid=2ab55feb-2a38-469f-8eac-b023c93ed25e-1&algo_pvid=2ab55feb-2a38-469f-8eac-b023c93ed25e&priceBeautifyAB=0
However you will be expected to wait up to 40 days.