how does the spring fit back?

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