Anytime there is a significant change in movement (decrease or increase) you should contact your dr.
Just like the other two comments stated I suggest to consult your Dr immediately. It's possible that she's just sleeping and growing, I had the same problem during that time (25 weeks). I couldn't feel my babies and would get nervous. But if there's substantial change in movement comparing to before then don't wait. Hopefully they can give you emergency ultrasound and check up on her. Were you hit hard in the stomach? :-(
My best suggestion is to go to the ER immediately!