Check your About dialog to see if LR is running in CC mode or 6.x mode. Account updates and/or monthly sign-in changes can confuse it.
If it is not running in CC mode, sign out and back in if a simple restart of the program does not solve it.
I had the same problem where I had used it before but it disappeared (even on an image I had used it one). Restarting lightroom fixed the problem.
Also you must be using the 2012 processing for the Dehaze filter to be active.