Broken Lawn Mower

I mowed the back on Saturday, and about half the front on Monday, but when I tried to finish the lawn today the mower wouldn't cooperate. The cord you pull to start it didn't have any tension to do whatever it is it does that starts the mower. I took it apart and saw a piece broken off, but I stripped the screw and couldn't get it down to its core elements. I still think I can make it faster and stronger, but I not w/o the help of an authorized Briggs and Stratton repair center.