The first thing to try, if you haven't already, is to turn the key and then push down on the cap, don't try to pull it up. Often times these gas tank caps open if you just push down. You want to avoid pulling on the key as that's how the keys get bent and ultimately broken.
If that doesn't work, you might consider just consulting with a locksmith. With the key not stuck in the lock, they might be able to get it unstuck so that you can use the gas tank. Then depending upon what the state of the lock is they can either replace it or just maintain it. But, if the key gets stuck in the lock or if they can't get the lock open, the only way to replace the lock is to drill it out and that's problematic.
EDIT: I guess one other thing to check is that the vents aren't clogged. If the fuel tank vents are clogged and the temperature has dipped you might find that the tank is being held closed by air pressure. That's especially problematic if the tank is nearly empty.