I cashed out a 401k worth about $40 k. $7500 was withheld in tax. Will I owe any more on it?

Depends on your overall income and tax situation, but most likely yes.

Maybe. It depends on what the rest of your taxable income is.

Most likely yes.

The $7500 is only an estimate and it normally falls short.

The normal withholding is 20%, so my guess is that you cashed out about $37,500

Federal:

22%  -  Typical tax bracket

10%  -  Penalty if you are not 59.5 years old

32%  -  Estimated Total Tax Rate

$37,500 x 32% = $12,000

$12,000 less $7,500 = $4500 additional owed.

State:No state taxes were withheld.  If you live in a state with an income tax, you estimate owing about $2000

$4500 + $2000 = Owing an additional $6500

Depends on your tax return, your filing status and your marginal income.  Assuming you file as a single person, you would pay 12% on every additional dollar up to about $40,000 taxable income (which is likely about $52,000 gross income given a standard deduction).  Over that, you would pay 22% on every additional dollar up to $84,000.

That is only Federal tax, you could have state tax on top of it.  You simply haven't provided the needed information to make even an educated guess.