On the main spring housing, that little lever that you pull down in order to open it, are you able to do that? if not, try pushing it up as you do it. If you manage to do that, your problem will be simple to fix. Like Luke said, there is that little pin that should be in the down position as you put in the main spring housing in.
If it was up, is probably what got stuck.
If it was up, is probably what got stuck.