Contact your former employer to ask them. They will direct you to the record keeper so that you may initiate a rollover.

If your balance was not small, like $1000 or less, it is almost certainly still sitting in the plan. If it was a small balance it could have been forced out of the plan into an IRA. Again, you'll need to ask your employer where that would have ended up.

Do you have the last statement you received on this 401(k) account? If you do, there may be an address and phone number on the statement you can contact to advise them you want to rollover the funds. Many times these types of plans have a plan administrator such as Principal Financial or ING or some other investment group. Your last statement would help you see if that is the case with your plan and if so, you could contact them and have them send you the paperwork to process a rollover.

The other option is to contat your previous employer, however if your plant has been closed down, you would most likely have to then contact a headquarter or corporate office.

Another option is to check with your state's unclaimed property office, or visit the free missingmoney website. (Avoid scam websites that want to charge you to do a search.)