How do I open the petrol flap on my Ford Focus?

The fuel flap is stuck so I cannot put petrol into my Ford Focus (Reg 55). I have had the car two years and this is the first time it has happened. Also, is this a common problem? I could be stranded miles from home so it is worrying in case it happens again. Even the breakdown companies won't help as it is not regarded as "immobilising the car" even though a car needs petrol in order to go anywhere!