Sounds like the fuel flap actuator may have failed; this was quite common on earlier mk7 Golfs.
Sometimes, repeatedly pressing the lock and unlock buttons on the key fob might release it. You may need to also insert something like a plastic credit card into the gap between the filler flap and the rear quarter panel and help to encourage it to open while pressing the key fob buttons (use a plastic card and not a metal object so you don’t damage the paintwork).
I‘m not aware of a way to manually open the flap, other than try the above method.