How can I find my father's 401k account now that he passed away?

My father passed away a year ago, but he never talked to us about his finances. On his last check stub, I noticed he had a 401k account. I haven't been able to find a statement of it. I went to his employer and they told me the company changed ownership and that it had nothing to do with the company that hired my father. What can I do to find this account?