What should you do when the Pull cord on push mower won't retract?

The coil spring that retracts the pull-cord is either broken or came loose from where it hooks up. You will need to remove the spool housing to repair/replace it. A repair manual form your public library can guide you through the steps to do the repair/replacement yourself if you are inclined to do so. There are several repair manuals available at the public library - perhaps there may be one available for your particular application.