When you withdrew money from your 401(k) that was a reportable event. (I said *reportable, not taxable).
If you put that money regardless of the amount since there is no limit, into another retirement vehicle such as an IRA and you do it within 60 days of withdrawal from the 401(k), then your reportable withdrawal event is not taxable and no fines are assessed.
If you put the money into a ROTH IRA, then the entire amount is taxable income, but no fines are assessed.
If you do not do either of the above, then the entire amount withdrawn is taxable income in the tax year of the withdrawal. If you are not of retirement age on the date of withdrawal, then the entire amount is subject to a 10% penalty in addition to the taxes you will pay on it also.
Note that if taxable, then it will be included as a part of your MAGI for the year and has the potential to put you into a significantly higher tax bracket for that tax year too. If your MAGI is high enough, you also lose many of the tax credits and other deductions you would otherwise qualify for. As an example, if you have a kid in college and your MAGI is over $160K for the tax year, then you don't qualify to take any of the education credits associated with any college expenses you may have paid for your dependent student.
