If you withdraw from your 401k plan before age 59 1/2, you are subject to additional 10% tax penalty on the withdrawal. You might be eligible for an exception, if that is the case, you would see a code 2, 3, or 4 in box 7 on your Form 1099-R (but most people do not get this; see Form 5329 Instructions, page 2, middle column for a list of exceptions when the additional 10% tax penalty on early 401k withdrawal does not apply).
You report your additional 10% tax penalty on early withdrawal on Form 1040, line 58. Some cases are required to file Form 5329 (for example if code 1 in box 7 on your 1099-R was missing).
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Tax on Early Distributions and for a list of exceptions too. Read instructions for Form 1040, line 58, and Form 5329. Sorry to hear about your company.