What should you do if you received notice you owe for a cancellation of debt from your ex-husband's bankruptcy and it has nothing to do with you and you never filed bankruptcy?

You really need to find out exactly what you are being noticed about. It may not be what you think, especially as the bankruptcy probably eliminated his debt. If you were also liable on that debt, you could end up with Cancellation of Debt income to report on your income tax...and receive a 1099C But you may also be simply subject to a clerical error, (someone thinking your ex is your current, or anything). But until you know the specifics, everything is speculation. If after speaking to them, you feel you were never responsible for the debt, send a certified letter saying so (any reasons you have doesn't hurt), that they cease any collection efforts and requesting they provide copies of specific documentation of why they believe you are liable. If forgeries or the like are found, proceed as needed.