A company has three ways of dealing with a 401(k) for former employees:
1) For the smallest accounts, they can send a check for the vested balance a few months after your separation from service.
2) For slightly larger accounts, they can automatically transfer the account into an IRA in your name - the details should be spelled out in the Summary Plan Description.
3) For larger accounts, they simply keep it invested in the 401(k) as it was the day you left. It will remain there until you give further instruction.
Contacting the HR department should allow you to get the name and contact information for the 401(k) Plan Sponsor. If your account is still in the 401(k), you'll have to decide whether you want to stay or go.
You need to contact HR immediately and rollover the money you had.
You should still be getting statements, either in the mail or electronically. If you had an online account with your 401k custodian, then try logging in again. The account should still be active.
What Jeremy says about the 401k is correct. The custodian for your plan is Charles Schwab Retirement Services and you can try logging in or contacting them here: https://www.schwabplan.com/net2/cmspages/participant/occ/index2.html
Sounds like this is another example of why journalists should be encouraging people to rollover retirementsplans to say a Fidelity or Vanguard as people leave their jobs.