42 in. troy built mower deck help

my deck belt stopped turning the 2 blades . took deck off --a spring to the pulley break bracket had broken . have bought a new spring but cant figure out how just by replacing that spring would engage the idler pulley which puts tension on the deck belt to make the blades turn. What am I nmissing here ? If anyone could help with this & nees pictures of what I am talking about ,I will attempt to send . :confused: