If you hold the bottom side of the disk to the light you should be able to see if there's a smudge or scratch. If it's a smudge, cleaning the disk will often fix the problem. I like to use the microfiber cloths used to clean eyeglasses or, short of that, a tissue. Just rub the smudged spot. (When using a tissue I generally steam up the spot with my breath first.) Don't exert more force than necessary; it's a flimsy disk, not a kitchen sink. If the disk looks good, put it back into the Wii and see what happens.
A scratch is more problematic. If it's a game you just bought, try exchanging it where you bought it. Otherwise you can try polishing out the scratch. You can find a good explanation of dealing with scratches at WikiHow.
Some older Wii units also have trouble with dual layer discs, which pack more info onto a disk (Super Smash Bros. Brawl and Metroid Prime use dual layer discs). If your Wii is having trouble reading a dual layer disc you can try Nintendo's Wii lens cleaning kit.
If after all that, your Wii still can't read the disc, it's probably just a bad disc.
