All lines dead: it is electrical, not the glass
If nothing clears at all, the grid itself is probably fine. Look at the fuse, the relay, the switch, and — most commonly — the two tabs where the wires connect to the glass at each side.
Those tabs are soldered or bonded to the glass and they work loose over time, especially if something has been pushed against them in the trunk. Reattaching a tab is a cheap fix.
One or two lines dead: the grid is damaged
A single stripe not clearing means that specific conductive line is broken somewhere along its length. Look closely for a scratch crossing it.
Causes, in order of frequency: something rigid slid against the glass in the back; ice scraped off the inside; an abrasive cloth or a paper towel used with pressure; or tint removed with a blade instead of steam.
Repair kits containing conductive paint exist and work reasonably for a single clean break. They are visible up close but functional.
Prevention is the real answer
Never scrape the inside of a rear window. Never use anything more abrasive than microfibre on it. Do not let cargo rest against it. And have tint removed with steam, by someone who knows the grid is there.
A comprehensively destroyed grid is not repairable in any satisfying way — it means a new rear window, which is why we treat tint removal on the back glass as careful work rather than a quick job.
Need this done rather than explained? There is no shop to drive to. We come to wherever your car already is — your driveway, your workplace car park, an apartment lot, or the side of the road it stopped on — anywhere in greater Sacramento, with no call-out fee and no mileage charge. Give us the year, make and model and you will have a firm price before the call ends.