What happened to free single of the week?

What country's Store are you using? In the US, the button for the Wombats free single is right in the middle for me. You can also click on the Music link under the iTunes STORE header, then look down the left side to see all three free tracks.

I am also having this issue. It is the Canadian Music store. I have looked all over can not find the single of the week

I could be wrong, but I don't believe the Canadian iTunes Store offers free singles. Not all countries' iTunes Stores do; Apple's apparently not been able to work out the necessary arrangements with all the various entities for all countries.

It did exist. I discovered so many new artists through that litle banner. Its not there anymore though.

They don't have a section for free tracks as of today, so if the Canadian iTunes Store did at one time, then apparently they've ceased to offer them at least temporarily. It may be that they just couldn't negotiate anything with a record label for this week and with luck there will be a new free track in the future. We have no way of knowing. though.