This is a difficult one - more so than usual.
Without the make and model and without some indication of whereabouts on the mower the spring fits and what it does - it's pretty much a wild guess.
A rotating spring - a compressed spring or an extended spring? You do not say.
You have the advantage because you can see the spring - you can tell how long it is and you know the approximate place where it should be fitted and the job it does.
Perhaps there's a hole or an anchor of some sort. Use your powers of observation.
You may think this answer is a bit flippant and I suppose it is - your fault for being so vague - don't you think?
Good Luck..., October 2015
