I got our wii for xmas - 2 days ago and had exactly the problem you described!!
I fixed it by "smacking" the wii i.e. a short but hard hit on the side.
I didn't smack it on the "back" - which I did try, but it didn't work.
Hitting it on the side twice - quite hard did the trick!!
I am still disappointed it has gone wrong so quickly!!
Happy new year,