As the title states I've got a (seemingly) seized fork.
Thing is I'm new to this particular headset and unsure wheter it needs unscrewing in order for the stem and fork to seperate.
Obviously I've loosened the allen bolt.

I tried to attach photo's, but upon submitting it tells me I'm not allowed to post links ,
so I created an imgur post: just go to imgur and add this to the url: /a/ThqAlsT

Thanks!

Loosen the hex bolt a few turns. Don't remove it all the way. Then smack it smartly down its long axis with a plastic mallet to dislodge the wedge nut. Then the stem should be able to be twisted out.

Good video to remove fork and service bearings

Assuming that you are talking about not being able to get the stem (the thing that the handlebars attach to) out of the fork (more specifically, the fork steerer tube).

If so: The allen bolt you loosened was the proper first step. At the bottom of that bolt it screws into either a wedge or a tapered plug. When you tighten the bolt, it pulls the wedge up and fixes the stem in the fork steerer tube by pushing the matchine wedge surface on the stem against the tube. If you have a plug-type, then the plug bears on internally tapered surfaces in the stem and push them out, wedging the stem in the tube.

In either case, the most common technique is to slightly loosen the allen bolt, and tap it with a hammer (the purist will say "use a brass punch aka a drift, in between the hammer and the bolt to avoid marring the bolt head). If you do this a couple of times (slightly loosen, then tap) the wedge or plug will disengage and you'll be able to pull the stem out.

If the fork is seized in the frame and you can't turn it, that's another problem. If that's the problem you face, let us know. Good luck.

Thanks guys, I smacked the allen bolt and the wedge nut came loose, after that it took half an our of twisting and pulling but I got the stem out eventually.

Cheers.