In our 3+ years of Leaf ownership there have been at least a dozen occasions where the car was parked in the unheated garage after being driven in the rain, followed by a temperature drop that caused the charge door to freeze shut.
As suggested above, warm water is your best friend. You might also consider a heat gun with a low setting, either corded or battery-powered depending on your needs and usage. On other occasions when we didn't need to charge immediately, I've pulled the frozen Leaf into the sun which is often enough to free the door, even in below-freezing air temperatures.
Now that I know the conditions that cause this issue I try to be more proactive by popping the charge door before it has the chance to freeze shut.