The cable may be broken or stretched, they may be an emergency release in the boot behind the trim adjacent to the filler, have a look there.

Remove the trim as suggested above and find the motor (see attached image).

You can pull the rod towards the motor to open the filler flap.

Disconnect the electrical connector from the motor and attach a test light across the harness plug... have someone pull the fuel door switch
and see if the light "lights up". If yes, replace the motor. If no... check a wiring diagram for fuse # and wiring.

Red circle is the motor, green arrow is the harness plug connection point.

Thomas