If you were not legally married on 12/31/2015 then the tax return for your ex is his alone.
If you were over the age of 18 and not a full time student under the age of 24, had gross income in 2015 of $4,000 or more then no one can claim you as a dependent. Even if you were legally married on 12/31/2015 a spouse cannot be claimed as a dependent.
If not legally married, your filing status should be Single or if you have a qualified person as a dependent on your tax return then Head of Household . If he has filed a tax return with your Social Security number entered as a dependent and the return has already been accepted by the IRS, then you will not be able to e-file your tax return. You will have to print and mail your tax return. If you are owed a tax refund the IRS will send you the refund. However, within a year the IRS will contact you and your ex to ascertain whom should have claimed your personal exemption. The one failing to prove they were able to claim your exemption will have to pay back any refund based on the exemption plus paying additional penalties and interest.