Depending on the threadless headset there may be a wedge ring pressed down into a groove on the inside diameter of the top race between it and the steerer. See if you can use a pick of some sort to pry it up, making it easier to remove and the fork should drop out when it is no longer retained by the pressure of that ring. A hard enough whack should move it regardless.

It's the centering sleeve in this photo but yours might be different.