This is a common issue if you don't know that momentarily cycling the power button runs you thru all the brightness levels. Happened to me. Just quickly press power button until full brightness, and leave it there. From the manual:

Adjusting the Backlight


  • Select Settings > System > Display > Backlight.
  • Adjust the backlight.TIP: From any screen, press repeatedly to scroll through the brightness levels. This can be helpful when the brightness is so low you cannot see the screen.