Its a bit unclear what you did or what the problem is. I presume you are having problems reinserting the seat post into the frame. From the above diagram, the seat post slots into the seat tube, and is held in place by the seat post clamp (not labeled).
I also presume hex bolt 'threaded' means you cannot undo the bolt in the seat post clamp. This clamp has to come off to insert the seat post. My first attempt would be to try to knock it off with a hammer and screw driver by tapping gently around the circumference and 'walking' it up the seat tube. A bit of heat might help. However if the bolt is tight I would not bother.
If that does not work you should be able to cut the bolt with a hack saw blade in the slot, and remove the clamp. An alternate is to drill the bolt head off or cut the clamp itself. New clamps are readily available so do not worry if you damage it.
Once the clamp is off, the seat post should slide fairly easily into the frame, set the height and tighten the new clamp (not too much).