The connector may be the problem (not likely, though). Adjust the mirror to its outboard extreme, and peak through the gap to see if the connector has come loose. If it has, use needle-nose pliars to replace it.

The most likely problem is that the heating element has failed, and you must replace the mirror to fix it. The mirror simply snaps off from a plastic connector. From the same outboard position, put your hand in the gap and pull firmly. This move takes some confidence, but it works. And since you are buying a new mirror glass, no need to worry about breaking the old one anyway. The new mirror just snaps on the same way with a little pressure.

Gerald C