Strange one this. Couldn't place medium wheatfields yesterday, so had to make do with the humble wheatfield instead hoping that it would be ok today. It isn't.
Have tried the usual things, yesterday and today, ctrl-f5 refresh, log off/on etc. Still doesn't work.
It can be selected from the menu and any available plot is illuminated as usual, but the graphic doesn't show up at the cursor position and if you click to attempt a placement nothing happens!
It's just the medium wheatfield, all other buildings seem ok (I went through them just to check).
EDIT: As an extra test, I've just tried settlers in another browser (Chrome) and guess what, all is fine! Going back to IE10 sees the problem return. Looks like a browser problem then