all of a sudden the key will only go a third way into the ignition and i dont want to force it and break it. It is a 2002 sedan de ville? How should I proceed? The car is in park and never had this problem before.

Might be something jarred loose and fallen down in the path of the key, or some foreign object has found its way in there. Does there appear to be anything broken off the key? A part might have broken off and lodged itself in the cylinder.

This is most likely a failed lock cylinder.

A lock is made up of driver pins, that push tumbler pins, that are pushed on to the key by tumbler springs. Very unusual for one to break, and if they do they generally won't stop the key from going in. Is there a possibility somethere foreign has been pushed into the lock.

In any case, pushing on the key will not clear the obstruction. You probably have to take it into a locksmith, and/or replace the ignition switch.

Maybe with a small tight bend on a paper clip you can pull the obstruction out.

I have repaired several GM lock cylinders because of a tumbler wearing out.

They usually get stiff then seize up though.

How heavy is the key chain? Big-heavy things hanging off the cylinder is a no-no.

Is it possible someone has put debris in the key slot?