Hey mate, have you tried inserting different discs? Sometimes this issue can be caused by one faulty disk. If not, you might have a disc drive read error.
This is almost always a hardware issue. If the Wii is now out of warranty, this is what you can do, and works most times. Open the Wii up and remove the disc drive. Now open the disc drive casing and check for any foreign objects in it (e.g. coins, bugs, plastic etc). Next, clean the disc drive with compressed air and also clean the laser lens. To do this, first spray it with compressed air to remove any large build ups of dust etc. Next you'll need to use a cotten bud and rubbing/isopropyl alcohol, and clean the lens that way.
This guide should help you do this repair Nintendo Wii DVD Drive Lens Replacement
Any questions, feel free to ask.