We just went through this at my work. My bosses father had a life policy that had been sold by the original insurance company to a new company. Original company said we had to talk to the new company. New company said they couldn't locate the policy by the policy number of the old company. Of course we had no way of knowing the new number that the new company had converted the policy to. Round & round we went.
We finally resolved this by contacting the office of the state insurance commissioner. Come to find out, all of the insurance companies have a unit that does nothing but track down lost policies. Through the insurance commissioner we got the contact information needed to get to the right department of the new company. Bingo! Everything is resolved & the policy is no longer "missing".
So - your best bet is to go straight to the insurance commissioner for your state. Good luck!