they are always the first thing to break..... take it off the bike, the spring is probably poped out... you will see it right away e hex nuts off and remove it...remove the handle from the rope, undo the hex bolt inside, remove the spindle (round plastic piece the rope is tied around. now the spring is going to completely undo itself from the spindle(the spring is a flat piece of metal roughly 6 feet long that is rolled inside the plastic spindle).simply reroll into the spindle, making sure it rebounds the rope and doesent expel it from thee housing... re assemble the center hex with washers pawl spring and plastic fitting,and tighten it good... put the rope through the hole of the pullstart housing, then back through the handle... voila....
youtube has ALOT of videos about taking it apart and putting it back together, and you can find a tutorial on these forums too...