OP, you have put your bike into gear 0. It's because you were on a steep hill, and you had to downshift in order to keep moving because you are new to road cycling & just building strength. In gear 0 your pedals will just spin. It's like neutral on your car, but completely different. Next time this happens you should try to shift both front & rear gears at the same time, vigorously. You have to do them both at the exact same time or it won't work, you might even put it in reverse. If that doesn't work, call your LBS to come out & give your bike a boost. Sometimes it's easier to get out of gear zero by upshifting while going back down the steep hill (going forwards of course).