My TV has lost brightness to about 80% of the screen?

My TV has lost brightness to about 80% of the screen?

My 55” Toshiba has a line going horizontally across about 80% of the way down the screen. Everything below the line is normal brightness and color, whereas the top portion of the screen is very dark. Happened randomly after not being in use for 7 days. Powering on and off the TV and all devices did nothing. The line/box is there even when no devices are plugged and appears on every input.

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