Your lack of specifying which car and engine you have may have caused people to be wary of venturing an answer....

I think it is great that you changed the woodruff key.... that is SOP for chain maintenance even though there are several who claim otherwise... ( but they did not make these cars or write the manual )....

Since you did not change to a new ( shorter due to less wear ) chain you may not need to do anything to the tensioner.

To reset it you must take it off and push the rod out the end closest to the chain...and reinstall from the outside.... it is a one way function.

There may be two designs of tensioners... I am only describing the one I know of... the only one shown in my 616,617 books.... But Larry says his 617 has a different style... so further research is needed....
But I think you are ok since you did not change the length of the chain...

It works with a combination of oil pressure, spring tension, and ratchets... Perhaps later they decided that the spring... which was only a " don't back up" function was not needed... or maybe yours fell out with previous mechanic and he did not see or care....