Do you mean 0 because you had taxes or or do you mean 0 because you have not yet received the tax refund,
If you had taxes owed then Line 18 on the Form 1040-X should be blank (empty) and the amount of taxes owed and PAID should be entered on Line 16.
If you filed for a tax refund and have not yet received the refund then you need to wait for the refund to be paid before amending the tax return,