My car had ridden smoothly for about 350 miles today, with no warning lights or odd noises/shimmies.?

I went in a store, came back to start it. It made a grinding, then a popping sound, followed by a burning smell. It turns over, but it continues to make the horrible popping sound and feels as if energy is surging and waning. It's a 2003 Ford Expedition that was just given a clean bill of health by my mechanic two weeks ago. What could it possibly be (aside from "nothing good" lol)?