The LCD screen connector may not be fully connected. It's possible for this to happen during normal use, but it usually happens right after doing a repair on the tablet. The only way to fix this is to take the tablet apart and push the LCD screen connector all the way into the port.
Follow the Battery Replacement guide until you get the tablet's back cover off - there's no need to remove the battery. Then remove and re-insert the LCD screen connector, making sure to push it in all the way. Then replace the back cover of the tablet.
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