Engine start, stop, start, stop...

Something weird happened this morning. Temperature about 55 degrees. I moved the Prius from the garage to the barn to load things in the hatchback. Then I drove about 200 feet back to the garage and shut off the engine. The heat was set to about 75 degrees and the engine hadn't turned off since I first started it.

When I pushed the power button, the engine started and stopped and started and stopped, over and over again. I tried depressing the throttle and nothing changed. I tried to shift into reverse and it the vehicle wouldn't move. So I pressed the power button and shut the vehicle down. Then I pressed the power button again, lowered the temp setting and all was fine. I drove the vehicle for about 30 miles parking it and turning it off and on several times.

What was the cause of the stopping and starting?