Not had this problem, but have you been filling ok until now or is the bike new to you? try pushing the key into the lock, it has a slight springy resistance to the filler cap.
bought it new in 2008. its been working fine , but it has been sitting up all summer, im guessing its the humidity got kinda rusted up , but if it wont open I don't know what to do other then drill it out, been working key in and out and letting it soak with some oil,i might have to take off tank and go in from underneath, any thoghts
I know the right person will see this post and have an answere, or different ideas, my next option is to destroy the whole cap and install different one ,