Contact the Human Resources department at the old job. They will have information on the status of the plan or a previous plan and what has happened to your account.
It used to that you could leave a small balance in an employer’s 401 plan and it would continue to be part of the plan until such time as you pulled it out. A few years back the IRS changed the rules and employers (or plan sponsors) are permitted to separate any accounts with a small balance ( I believe it is either $3500 or $5000) from the plan. Your money will now be maintained in a separate account at the same fiduciary institution or investment company as before.
Now once you find your account you should “roll it over” into a Rollover IRA account with no fees as you can find at Fidelity or Vanguard or T. Rowe Price or another investment company or you can roll it over into your current employer’s plan if that is permitted under the rules of the plan.
Make sure you do a direct rollover with the balance being transferred from the plan to an IRA account and that you do not take possession of the funds. I don’t want to overcomplicate this with an explanation of the issue but doing it this way will avoid any withholding from the balance and will avoid having any income tax or withholding problems.
You should be receiving quarterly statements from the administrator of the plan. The plan sponsor (your forment employer) can clear this up for you very quickly.
SRM