I had the same problem -- found a good solution on another forum -- http://www.htconeforum.com/forum/htc-one-s-help/414-missing-keyboard.html
I have the same issue. I just downloaded another keyboard to use Swiftkey 3.
The issue seems to occur sporadically and only in landscape mode. Instead of seeing the text box and keyboard you see two text boxes. However, if you press the screen in the lower text box it types letters as if the keyboard was actually there. The keyboard is fine in portrait mode. I have in the past been able to restart the phone and get the keyboard back in landscape, but today even that didn't work.