It should come off easily... I guess it has just been screwed too tight, but that is a guess. You checked whether it doesn't have a dent, which could block it, did you? Have a look on the inside from the top as well!
As I never experienced that, I won't tell you to use brutal force though. The choice is up to you, if you want to try or not. You can also give the light back and ask for a new one. If you kill yours, this won't be possible anymore. Meanwhile change the cells from the top...