Try going to the Mii Channel and create a new Mii (how to).
Once the Mii is created, verify that it shows up in the Mii plaza.
Power off the console, then power it back on and go back to the Mii channel.
If the Mii you just created is still there, then the Wii console's system memory is fine and a repair is not necessary.
If the Mii you just created is not there, then there appears to be a problem with the Wii console's internal save feature and a repair is necessary. Please click here to get in contact with our Consumer Support department