In reply to elliot.baker:
> the thing I dropped wasn't that heavy.... so you're saying I might have made the ceiling move away from the joists a bit then?
> I'm worried if I start screwing things in I'll make cracks or something or make it worse.
Get back in the loft and if the board is still tight against the joists, just fill the holes.
Otherwise, do as JLS said and re-fix the board (with the black trumpet headed plasterboard screws).
Drive them in so they sink into the board without puncturing the backing paper i.e. not too far!