a few things to try
is the safety OFF?
is the hammer down?
is the take down lever still in the open position?
did you put a mag in? if so remove it
palm slap the butt like you were inserting a magazine (just incase something has slipped)
now firmly grip the slide and pull forward
if not cock the hammer, see if it releases the slide?
if the trigger will move, try to ease the slide forward with the trigger depressed.