Replacing cord to draw curtains?

Cord has broken to draw curtains - a length of cord hangs down on one side that closes the curtains when pulled but the only way to re-open them is to climb a stepladder and do it manually from the top. I can't quite see what I need to do to repair or replace the cord and would like to know if I could get a diagram of how to do this. The draw cord is only on one side of the curtains and used to be in a loop so that pulling on one side or the other would open or close the curtains - but now there is just the unlooped cord hanging down.