If you turned down a job offer a few months ago how would you tell the employer that you want the job now even though the position may not be there?

Answer

It sounds like you answered your own question. If the jobopening was a few months ago, chances are, the job has been filled. If the job is no longer there, you have to chalk it up to a missed opportunity.

If the employer is not someone you know (or someone you can call "out of the blue"), send another resume and cover letter to the employer (don't assume they remember you specifically), explaining your situation, your regret over not having accepted the job and explain (but not in too much detail if it was too personal) the reason why you declined the offer and why you've changed your mind. Ask to be considered for any future similar positions with the company. Obviously if that position is still available and they're still interested in you, they'll let you know. If not, you won't feel rejected and they'll at least keep you in mind.