The disc may be too scratched up for the console. Try to clean the Disc with a microfiber cloth (or something that won't scratch it) and use a dvd lens cleaner on it to remove smudges. If the disc is in good condition, then the laser on the Wii may be wearing out. That means that you will have to send the Wii to Nintendo to get it fixed or find someone who has Wii parts and knows how to take apart and repair a Wii.
This is usually a problem with either the laser inside of the Wii's drive, or some other issue going on inside of the DVD drive. The following website sells the DVD drive mechanisms and does repairs on the Wii as well:
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