Hmmm...has the bike been to the shop recently?? They will often extend the seatpost so that it will fit in a clamp for them to work on it.
Where the seatpost enters into the tube of the bike, there should either be a quick release or a bolt. You need to loosen that bolt and adjust the seatpost. Make sure your saddle is straight when you're done
That's all there is to it!
Carrie Anne