If you inherited the 401(k) (or part of one), then you are the account holder of a beneficiary 401(k). Your withdrawals are subject to regular income tax, but not the 10% penalty for early withdrawal, even if you are under age 59-1/2. You also have the option of rolling over the money into a beneficiary IRA, if you don't like the investment choices in the 401(k). This is because no tax was paid on the contributions, so withdrawals are always taxed.
If the 401(k) account includes a Roth option account, the withdrawals would be tax-free as in a Roth IRA.
You should get a 1099-R from the plan administrator, if you didn't get one, check your account online or call the plan administrator.
In the rare event there was no beneficiary, the plan balance would have been paid to the estate, and the estate would pass the money on to you. I don't have the tax consequences of that at my fingertips--it may be that you owe the income tax or that the estate paid the tax for you and you just inherited cash. You would need to check with the executor of the estate.
