Look at the shaft - there should be a notch in it.
Then look at the damper hub, another notch.
The key is a long square thing. About 1/4 X 1/4 X 1". Then when you line the 2 notches up the the key slides into the hole made by lining up the 2 notches. It locks the 2 parts together.
Then the bolt and washer goes on, trapping the key in the hole and holding the damper from sliding off.
When you pulled the damper off, that key probably fell somewhere.