In some cases, you can leave your 401(k) balance with the plan even after you separate, so that you can grow your retirement. Sometimes you can roll it over into the plan at your new employer.
If you took a simple withdrawal, you had 60 days to contribute it to an IRA as a tax-free rollover contribution, to grow your retirement savings. If you kept the money, you owe income tax, plus a 10% penalty if you are under age 55.