Wide Black line down centre of screen
Broad black line down the centre of image.
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If this is only on the screen including menu screens, and not on photos you take then it could be either the LCD or the video driver. Either way, your best bet is to put a watch on eBay for a camera of the same model for spares or repair but with a good screen. There are several repair guides at Nikon Coolpix S3500 Repair from which you can quite easily swap out the LCD. If that doesn't fix it, swapping the main logic board is only one step further.
However, if you also have the black line on photos you take, it'll probably be a problem with the image sensor, though if you're very lucky you might be able to fix it just by reseating the ribbon which connects the sensor to the main logic board. Again, your best bet in that case is to swap out the lens assembly from a "spares or repair" unit, though many advertised are likely to themselves have lens faults. In principle you could swap out the sensor, but normally it's best left in place. You only need to get the tiniest amount of dust on it to show up on your images, and in manufacture, special test equipment is used to ensure that it's not even at the slightest angle to the focal plane.
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