Well, first of all some background info would be nice.
1.) How old is your Wii?
2.) Have you accidently pushed it off a table or similar accidents?
3.) Have you modified it in any way? (Homebrew or opened the Wii yourself etc.)
4.) Is it one particular game or is it all of yours?
5.) How long has this been occuring?
6.) Was the last game you played Super Smash Bros. Brawl or Call of Duty: Black Ops? (Apparantly these 2 games are dual layered which some Wii's can't read, but luckily Nintendo fix that problem for free. only that problem though. Unless your Wii is in warranty.)
7.) If your Wii is new, where did you get it from?
8.) Is your Wii in warranty?
9.) Is your Wii wired up to your TV correctly?
If your Wii is in warranty, you can have it fixed for free. If not it costs about $60 to fix. Also if your Wii is new you can take it back to the place you bought it and tell them the problem and trade it for another Wii. Only if it's in warranty though. Also you break your warranty IF you have modified it.
Have a nice day.