The light sensor thing is a common feature of Sharp TVs, likely normal. It tries to match the brightness of the panel to the room's lighting.
When part of the screen doesn't work usually an internal problem. It can be as simple as the connector being loose internally. Common failures on TVs are capacitors related to the power system.
With an exact model you can often look for capacitors kits sold on ebay and amazon. You do need a soldering iron and the willingness to take the TV apart.
Other than that, contact your local TV repair center. There are still people out there who will diagnose and repair issues.
My usual recommendation is to buy a new TV, but a 75" model will be worth trying to save.