You installed your headset incorrectly...it seems like the wrong parts being forced together might have caused it to bind up. Try to shove a flathead in the small gap between the headset and the top seal and hit the side with a hammer to try to kind of pry it up.
This happened all the time when I had threaded compression with a thread less headset, and it still happens sometimes. I just hit the top of my fork as hard as I could with a big hammer, then once it slipped up a little bit I pried the parts apart.
Put your deck on something flat stand on it, and hit the top of the forktube as hard as you can with a hammer, you could probably clamp it to something to, as long as the forks are free to fall. That how I used to do it with my bike.
If you have an unsealed headset then put something under the fork because the force of the hammer hitting the fork could knock all the balls out of the bottom cage.
Here is the easy way to do it. first is there a little gap in the compression ring. second if there is get a small flat head screwdriver to fit in the gap and get a hammer and hit it on an angle then the compression ring should move which will loosen it up good luck with this method.