If you had less than $5k in the account, the firm that managed it could have forced you to tkae the money or roll it over, more than $5k they can't do this
Since it has been 10 yr with no activity from you is long enough for it to be escheated; IE stolen from you and put into the tax base
Call the employer and find out what firm manages it and start calling, it could have been taken over by more than 1 firm in 10 yrs

If you did not roll this money over to an account of your own when you left - it is not good news. If you do not get at least a yearly statement of such an account that another party is holding for you - it is worse news. I would start with the employer and the firm that you knew managed the account.

The employer does not hold the account, a financial institution does. Ask the employer who sets up their 401k employee accounts (or who did when you were employed).

You just need to find out who holds the account. That will be a third party. Call your old employer and ask them. Then you'll have to call the company that holds the account and give them your information. Should be easy.