If its the lock that won't release, try getting a flat plastic object and place it in the crack of the rear part of the charge port door. The black locking mechanism is on the rear inside facing part of the charge port and a little favored towards the bottom. If you can get the plastic piece in there just right you may be able to manually retract the (black) locking shaft to release the charge port door if its stuck.
All this input was very helpful, folks. And gave me the courage to apply a little more elbow grease and, hey presto, as if by magic, it is open.
Before receiving the wisdom of the group, I was worried about pulling too hard, snapping some sort of latch, and ending up breaking something which meant I couldn't charge even if I got the flap to open.
Going forwards, I will grease with Vaseline when I can, to make the mechanism operate a little more smoothly, like it used to.
Thanks again, all. Problem solved (easily and without cost: the best way).
Take it over to the Service Center and have them tweak it. I had this same experience two days after I got the car. At night, in the rain, at the the Cheyenne SC on my first road trip. Called the nice people at the Tesla help line, they said you could encourage it to open by pushing on the back of the door while clicking on the open-sesame button on the charger. That finally worked, but I had it in at the Service Center for adjustment the second I got back. It's been fine since.