Unless you see them pick it up (or someone tells you that they saw them pick it up) you lost your ball.
If you see them then you place a ball where your ball was.

If you are declaring it unplayable you can take two clublengths not nearer the hole, or you can go back as far as you like keeping the ball and the flag in line or you can replay the shot, if you were on the tee than you can tee it up again.