If you have an automatic, with the gearshift on the steering column, and an open driver’s side window…reach in and try shoving the lever into “Park.” Worked for me once, but the car was moving slowly, 5-10 mph. Instant stop. And nothing was broken.
FIAT= Fix It Again Tony
go for the hand brake or the main (service) brake.
if handy and car is not moving fast throw some big object under the tires - repeat if needed.
Oddly enough, this just happened to me over the weekend. My wife’s car (tiny Saturn) started rolling down our sloped driveway. I opened the door to pull the e-brake but the car’s momentum became too great and the door knocked me over before I could jump in. Fortunately, the ground levels out at the end of the driveway and the car stopped in a neighbor’s yard without doing damage to anything except me. I’m banged up and pretty damn embarrassed but at least nothing’s broken.
After reading the article that Myrr posted, I guess I’m pretty lucky. Next time, I’ll just let it roll. Those things pick up speed rather quickly.
What you should do is breathe a sigh of relief, having realized that you had parked with your steering wheel turned all the way. I was taught in driver’s ed when I was 16 to turn the wheels all the way to the side, the side depending on the direction of slope and the side I’m parked on, so that the car will run into the curb.
The failure of most drivers to do so is one of my pet driving peeves, second to the universal inability to turn into the correct lane when making a left turn.
(I will now get off my soapbox and hope that the demons of automotive accidents haven’t just realized they were overlooking me)
What? Why? Is she getting psychiatric help now?
A friend of mine had this happen to him once as he was trying to perform a wheel alignment once. I guess the brake popped off or he had forgotten to chock the wheels. Anyway, he started to try and stop the car from rolling off of the rack and the door hit him, and the front wheel ran over his two knees. He was so embarrassed, he jumped straight back up to see if anyone had seen it happen. The car came to rest on the other side of the shop, not damaging anything, and my friend was fine. When he first told me this I freaked out, but now when we recall it while drinking we howl quite loudly. 
As for the OP, I’d let it go if it was already too fast and use my extremely loud voice to scream at anyone downhill if possible.