Did you have priiloader or Bootmii installed....
I had my wii softmodded with the hackmii or bannerbomb (cant remember which) and my neice decided to try and update it to the latest firmware so it would be the same as hers without me knowing. But when it was updating the firmware the power when off. Now the wii just displays a black screen.
Can anyone give me any advice ??
If it failed midway through a System update then you'll not have much success with Priiloader or Preloader.
If you have bootmii/boot2 then you can recover it.
If you do not then you will need to return it to Nintendo most likely.
How old is the Wii?
Not really doing much Wii stuff these days.