Same as Sixer, I've also installed several nuheat mats, and you have to record all resistance readings several times - basically at each step of the installation -and call in the results so the manufacturer can be sure there is no problem that can be blamed on them. This is very helpful if a fault occurs, as you can avoid performing extra steps on a defective device that will need to be replaced anyway.
No resistance tells me that you are shorted out. I hate to be ther bearer of bad news, but it sounds to me like the tile needs to come up and the section replaced. If you try to fire it up, it will pop the breaker or burn up under the tile.
Good luck.
