Well, I am losing games. Don't know if someone dropped my disk, or what . . . but suddenly at the beginning of the snowball fight WF+ gives me a black screen and tells me it cannot read the disk. Take it out, clean it, restart - stops at the same point. Oh well, I can live without it. Now it is doing the same thing on another game on WF+. I am hoping it is just the disk. There are not any terrible scratches, but it consistently stopping at the same points. So, I guess my only option is to get a new disk. Unless someone knows a magic trick. Or tells me the same thing happened to them and it was not the disk . . .

Try to the Wii with the slot vertically as it was pictured in the Wii operations manual.
Maybe the lens in the CD player is a bit dusty and this can help cleaning it.

I'm sorry to hear about your problems with the disk. Do you have problems with other disks, that is with something other than WiiFit Plus? Step to the Beat, for instance?

As Methusalem said, it might be that the lens in the Wii needs cleaning. There were and perhaps still are kits available for this, but, as all they consisted of was a CD with a cleaning pad on it and a stick attached to the CD so that it could be moved back and forth, you could probably make something like that yourself: all you would need is a spare CD and a lint-free pad and some CD cleaning fluid.

If cleaning the lens doesn't work, then there is definitely a problem with the disk.