There are a couple of possibilities. One is the spring in the fuel door has broken. To test this have someone gently, very gently, try to pry open the door as you push the switch. or the switch or release solenoid is bad or has no power. In this case you have to probe the wiring to find out where and why there is no power. There should be a manual release in the trunk on the side of the fuel door. If you can,t see one you may have to peel back or remove the trunk liner on the fuel door side and manually move the solenoid or latch.