According to the IRS, Form 1120X, (Corporation Amended Tax Return) must be filed within 3 years after the date the corporation filed its original return or within 2 years after the date the corporation paid the tax (if filing a claim for a refund), whichever is later.
It is worth noting that a corporation tax return filed before the due date is considered filed on the due date. A Form 1120X based on an NOL carryback, a capital loss carryback, or general business credit carryback generally must be filed within 3 years after the due date (including extensions) of the return for the tax year of the NOL, capital loss, or unused credit.
Furthermore, a Form 1120X based on a bad debt or worthless security must be filed within 7 years after the due date of the return for the tax year in which the debt or security became worthless.
Thus, you would be entitled to file an amended tax return as you are within your statutory limit of the 3 year time limit. I would urge that you consult with your CPA who will be better able to file an amended 1120 X corporation tax return for you.