This is an example for a reason NOT to leave your 401k at an OLD employer ... an uninterested HR department in past employees. Hopefully you have an old statement for your old 401k (or remember who was managing the 401k if you don't have an old statement). Let's say the 401k was managed by Fidelity for example. Your statement should have a phone number how to get ahold of them. If you don't have a statement, but remember who managed the 401k, you can Google that management firm for their contact phone number. The management firm should be able to answer questions about your 401k account, once they've verified your information.
Your question doesn't include what you want to do with your 401k or why you tried HR. If you need help you may consider getting that from a disinterested third party adviser via NAPFA http://www.napfa.org/ and use the find and adviser near you.
Without more details, I could only speculate about the many options you have with your old 401k. Hopefully you're questions may be answered via the company managing the old 401k. Best wishes, Larry