Steve --

Sounds like a gouged grid.

If you have one line actually heating up, then you're delivering the required 12 volts to the grid. More than likely, something scraped against the rear glass creating a microscopic cut, or break in every line except the one that works. Because the cut interrupts the electrical circuit (in each line), no current will flow (in that line) so it doesn't become warm.

Climb back there and inspect a non-working line from end to end with a magnifying glass. I'd bet a cold one you'll find a tiny cut somewhere. Then look at the adjacent non-working grid in the same location -- bet you find another cut. And so on. Every line that is broken is, well, broken.

BUT!! There's a conductive glue or epoxy material available for just this repair. Another Brickster said Loctite makes this product. All you'd have to do is crap the insulating varnish off the line at the cut (both sides of the cut) and bridge the cut with a tiny line of epoxy, making sure you get good contact on both side of the cut. Then the entire line again becomes conductive and should warm up.