how can I fix the rear passenger door of my Ford Fiesta?

The rear passenger door of my car has been permanently locked for the past 3 months and it cannot be opened from either inside or outside of the vehicle. The door handle on the outside is loose and doesn't seem to be connected to anything on the inside. The interior panel has been taken off (eventually) and all seems okay but still the door won't open. It's like the latch has got stuck and won't release. The central locking locks everything as it should but still the door won't budge. Can anyone help please?