could be one of two things, you dont say what bike it is, so bit of a guess here, if your side stand has one of those safety switches to stop you riding off with it down, could be stuck open,remove the rubber cover if its got one &try a bit of wd40 into the plunger end to free it off. Or it also sounds like a badly dragging clutch, as soon as you put machine into gear it will stall, check your clutch cable free play is correct or check that any clutch push/pull rod or lever is not worn, does it bump start OK? if so, it could be the clutch dragging.
Chris P.Duck, September 2010
