How do I report a 401k contribution made last year that is not reported on my w-2?

You may contribute to a 401(k) plan only if your employer has a 401(k) plan set up for the employees. Otherwise, you cannot set up your own 401(k) plan unless you are self-employed (the so-called solo 401(k)).

It sounds like you have contributed to a traditional IRA account. Read the Form 5498, what does the form say? Is it an IRA account?

This was an IRA contribution, not a 401(k) contribution.  Traditional IRA contributions may or may not be deductible:  https://www.irs.gov/retirement-plans/ira-deduction-limits