it's possible a connector came loose. I would take it apart, and re-seat any connectors on the inside. If the problem still exists, then proceed with an LCD replacement.
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You can follow this guide to get to the connectors https://www.ifixit.com/Guide/Wii+U+GamePad+LCD+Display+Replacement/57152
it's possible a connector came loose. I would take it apart, and re-seat any connectors on the inside. If the problem still exists, then proceed with an LCD replacement.