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How old is your Nook?
Can anything be seen through the dark band?
How old is the Nook, and has it ever been dropped (onto a hard surface as opposed to a soft springy surface) ?
OK. Try holding down the Power button (or holding the Power switch, if it is a sliding switch) for at least 30 seconds. See if that reset does anything.
If it simply shuts off, then let it sit for at least 1/2 hour, to cool down and reset.
If it is Off after holding the power button for a while, just let it rest for a bit, and that will reset it.
Regrettably, your Nook may have a severe hardware problem, possibly a connector slipped loose. If it got damp or hot, that might have corroded a connector pin, or perhaps damaged the circuitry also.
Repairs will be needed, for it to work properly again. If you need further information, e.g. where to take it for repair or where to send it, let me know (and your local zip code also, I could use to look up info, if so.)