If the button isn't working, try to lock the doors with the key. Now, unlock the doors with the key and try the fuel door.
If that doesn't work the actuator for the fuel door is probably bad, does it click when you push the button?
There is a manual release for the fuel door on the passenger side of the car under the "a" pillar it should be a lever you can pull to open the fuel door.
Let me know how you make out!