HELP! My car won't start.........

Same problem! 92 riv w/ 70k on it. Starts up fine (or with only a moderate struggle) 95% of the time. However, as it did last night, sometimes it won't start at all. I've had, in the last week, two different no starts. First time, the engine was turning over but not catching, like it was out of gas or something. After about 15 mins of trying, I got it to start. Second time, I put the key in the ignition, turn it, and nothing happens. Silence. The lights dimmed a little, but NO movement under the hood.I let it sit for 15 mins, and, sure enough, it started. Both times I had recently driven the car for 10-15 mins. Crank sensor? Help...