There is a emergency rod release which can be accessed by removing the bootling lining panel on the driver's side. You just simply pull the rod and it releases the lock pin from the fuel falp and enbales it to open. If it is a faulty pin/mechanism then a bit of lubrication may do it good as a temporary fix.
Also, it could posisbly be that the pin lock housing unit requires a little bit of adjustment to re-align it correctly which can easily be done.