Bumping an old post because this is an ongoing problem with these TV’s. It’s not an isolated hardware failure. The backlight power supply is inadequate and shuts down when running an maximum brightness. This will typically happen in a bright room when the backlight control is set for “Auto Brightness Adjust”.
Other than return the TV and never buy anymore Toshiba products, you can try turning the backlight down to 80% (Press and hold the Home button until the screen menu pops up). That will give you a somewhat less bright picture to look at but at least you can see it.