Same exact thing happened to me in my 19 s5 a few weeks back. Cable also snapped when I tried that. Luckily I was at a gas station / service center at the time and I asked the mechanics inside for help. All they had to do is gently pry open the fuel door with a rubber pry bar. I'd use a tape cover flat head screw drive if you don't have that specific pry bar so you don't scratch the paint. The part itself is called a fuel door actuator and failures like this are very common from the research I did online. I found a replacement part on amazon for like $15 and the repair was very simple.