just relax
its hair after all! we've all gone through horrible hair cuts at one point or another.
dogs are the same.
there are different styles of cuts for different dogs so if you aren't specific i think they tend to cut it very short.
next time i'd bring pictures.
as for how the hair will grow, most likely silver because your dog is probably changing his hair colour from black to silver (all yorkies go through this) but they're still cute! i actually like my mickey with silver hair rather than black