Hi Andy. If your door shut behind you do you have what is known as a split spindle lock? When you put the handle down on the inside it will retract the latch. When you put the handle down outside the latch does not retract? You will need the key from outside to open the door. If you do not have the key then you will need a locksmith to open it for you or gain access by other means and open the door from inside.
Paul
March 2011