Well either it's broken, or more than likely it's gummed up. Try spraying it with a penetrating oil and see if you can free it up. Are your hinges good?
Edit to add; try the "search" function, you'll be amazed at what you can find on stuck doors.
seems it's common for the door to be locked when you think it's not due to the rod coming off the mechenisim. I've only had sticky door locks and not a completly stuck one so hopefully somebody else will chime in or you can find an answer in the archives.
[This message has been edited by DLCLK87GT (edited 01-14-2014).]