You file according to your marital status on December 31 of the year in question. If you were married as of 12/31/2017 and 12/31/2018, then you generally file as either married filing jointly or married filing separately. A joint return is one return, signed by both spouses, that lists all combined income, dependents, and deductions.
Did you and your spouse file a joint return in 2017 and 2018? If yes, did it contain all your combined income and deductions? If yes, you are finished, nothing further is required from you. If you filed a joint return but did not include your income, then you need to file an amended return for those years to add income and deductions that were originally omitted. If your spouse filed as married filing separately and only listed their income, and you did not file, but you had income, then you are probably required to file a return as married filing separately. If your spouse file a joint return and you did not know it because they concealed it from you, you may be in a special case, and we would need more information to understand.
However, it is odd that this is coming up 6 years late. What are you trying to accomplish and why has this come up now?
