Has any body have used SDL librabries,what are the possible reasons for SDL_SetVideoMode returning NULL ?

Has any body have used SDL librabries,what are the possible reasons
for SDL_SetVideoMode returning NULL SW_Surface ?

Has any body have used SDL librabries,what are the possible reasons
for SDL_SetVideoMode returning NULL SW_Surface ?

http://sdl.beuc.net/sdl.wiki/SDL_SetVideoMode

Thanks,got it the display was not initialised.