Not what you want : pls tell me in detail as to what you did and how you got it to a NON Working state...and can you confirm this problem under sunlight..try calling someone under sunlight and lemme know if the screen goes off... i would love such a problem in my N900 .. no seriously!! As for a fix : i guess rebooting..or enabling and disabling the proximity sensor again from ShortcutD .. or in the worst case reflashing...this cannot be a hardware prob unless you have dropped it or some physical damage has been done..