It has been my experience that when a 2 stroke engine seizes it does just what you are talking about, the piston seizes in the bore. Yes, often times when the engine cools down it is possible to "unseize" the motor. I have seen a few seize 4 stroke motors, most of them were Volvos but no all, and every one of them was seized up either at the cam or at the crank.

What you need to do is undo the timing belt and see if the cam turns, you stated that it did not. So we now know that it is seized at the cam to start with. Next you need to see if you can turn the crank, can you? If not you need to drop the pan out of the way and start pulling main bearings. Pull a bearing cap, inspect the damage to the shell, and try to spin the crank. Do this one bearing cap at a time until you can spin the motor. In the event that you have all of the main bearing caps out of the way and it still will not spin over you need to start pulling rod caps. Now because the rods will be positioned just however they were when it quit running you may or may not be able to access the nuts to pull the rod bearings.

We know the cam is seized and the crank won't move either. Let's entertain worst case scenario for a second. The crank is seized, the cam is seized, and the pistons have welded themselves to the block too. While fixing all of this in the car is doable you really need to put that motor in the trash and get a decent used engine. You will be both time and money ahead.
You can pull the trans, pull the head, pull the pan, pull the crank, and pull the pistons and rods all in your driveway. In my area there is a guy that will come to you, set up, and bore and align hone all four cylinders with the enigne block still in the car. You would need to find someone that does this, have him do it and then go back together with all new stuff in the bottom end. Get your crank cleaned up as I am sure you have metal transfer there, adress the rods, hopefully it is just new bearings and you did not spin any there, new pistons and rings. Get your head hot tanked and repair the seized cam situation and put the whole thing back together.
At this point we are talking ungodly amounts of time and money. If your crank and cam are seized I would go no further and decide if the car is worth dumping a used engine into or not, if it is then start making a few phone calls to locate a used motor and get on with pulling that engine.

Mark