Hit the H key while using the tool.
I really feel stupid here, but I found this post in one of the forums and have the exact problem: (I've tried the "H" key, but no luck)
When using the clone, healing, or hand tool on a zoomed in image, the cursor turns into the arrow pointer. The tools work on the outer perimeter of the image, but once the cursor moves to the inner part(I'd say 2/3's of the image) it turns into the pointer and can't be used.
I have had this happen a few times and a restart of Lightroom has always solved the problem. It seem like there is a phantom dialog box or something because you can resize this area sometimes by moving the pointer till it turns into a double arrow and doing a click and hold. I still see reports of this so it seems like a bug that has been in Lr for a long time but is difficult to fix as there is not a repeatable sequence to make it happen.