Remove the nut on the housing opposite the handles - spring is under there. you can order a replacement, but when mine went, I bought a mower recoil start spring for like $3 at True Value.
Just hook the lip of the spring through the retainer cap, slide the tab end of the spring into the split nut, and tighten it several turns to build return tension. There may be a pin stop on the DP body to hold the cap in place and preventing unwind - be careful! They can unwind/come off in a hurry!