If a golf ball sitting on the green, is knocked out of place by another ball coming onto the green, what happens to the ball?
You should have a marker in place of your ball when some one else is putting, etc. just in case of this. I guess just replace your ball where you remember it being.
The person whose shot struck the ball at rest plays his ball as it lies; the person whose ball was moved returns the ball to its original position.
There is no way you can be 100% sure of the exact placement of your ball again, so you play it where it lies
Play as it lies you cannot replace your ball where you thought it was