i hate doing this job. so annoying especially when it gets jammed.just replace it with a new star nut, punch the existing one out of you steerer tube with a long pipe (or old xc flat handlebars) these do work well, replace with new star nut (costs probably no more than $5) then punch it down carefully into the steerer tube trying not to destroy it or if you don't feel confident doing so, your lbs will do it for probably nothing.
Wouldn't it just be easier to replace the star nut. dont they only cost like $2 or something?where has it snapped? is it possible to twist it out somehow? (this will mean you only have to replace the bolt..)