When did your husband die and who was the executor of his will?

A 401(k) with a stated beneficiary isn't part of the probate estate. You would have to fight this one out with the 401(k) trustee -- did you keep any of the paperwork where you were changing the beneficiaries? Did you talk to anyone at the firm about the change? These are kind of feeble points, as the company can always argue that the change was considered but never made. You might want to consult an attorney.

As to the rolled over funds, that account would have its own beneficiary. You might have to dig through old tax returns to find where it was rolled to. He should have started getting Form 5498 annually from the trustee of the new account, showing the account value at the end of the year. If you can't find any copies of this form, try asking the IRS if they can send you an income transcript for your husband (1-800-829-1040, they will give you a hassle if you aren't the executor of the estate). I'm not certain if the IRS record will go back that far, and if he spent it soon after he had rolled it, there might not be a record.

Retirement money, such as 401(k) or IRA, only becomes part of the decedent's probate estate if there is no named beneficiary (or no surviving beneficiary).