Known problem on modern Kia vehicles - 

https://www.kiaownersclub.co.uk/threads/fuel-lid-not-working.38980/

Have you already had the dealer check it under warranty?

Gas cap doors usually have a cable into the passenger compartment, connected to a spring-loaded clip at the side of the door opposite the hinge. I would try using a needle-nose pliers to pull the cable to see if that would release the door. If that didn't work, a stiff but flexible piece of plastic might work on the spring-loaded clip. Hopefully you have enough gas to take it to a dealer for the warranty work,

A lot of vehicles have an emergency release in the trunk area near the gas cap door, check and see if you have one there.

If you get really desperate and the emergency cable release in the trunk doesn't work for whatever reason, from my experience the pins that hold the cap door closed are just plastic and can easily be busted with a screw driver or plastic wedge (like a plastic wedge set for removing interior parts) prying on the door (which is why I've found those "locking" doors to be useless in terms of preventing gas theft), or perhaps you can devise a wedge out of something thin that you can wiggle in and depress the pin enough to open the cap.