If the account was in your name under your social security number, I don't see how another person could have cashed it out... the IRS would have gotten notice of it under your SSN and come after you for the tax and penalty on it.

I would imagine that the 401k was through a benefits company or administrator that your former employer hired. If you know who the administrator was, you should be able to get the information from them. Do you know who the funds were through? John Hancock, Edward Jones, Ameritrade or possibly a bank? You or the employer would have had to authorize the cancellation of the fund. I seriously doubt they could "cash it out" without your knowledge as that is theft. My thought would be to call the US Department of Labor and see if they can steer you in the right direction. Here's the link to their website on the subject of 401k plans: http://www.dol.gov/ebsa/consumer_info_pension.html

Good luck!