The general rule is that you cannot amend from separate returns to a joint return after the due date of the return. (Separate returns means anything other than joint, not just married filing separately.) There might be an exception to the rule if your head of household status was not valid, in other words, if you did not qualify to file as head of household. I don't know if there could be an exception because your spouse has not filed a return. If no exception applies, your spouse has to file a separate return. You should consult a tax professional to find out what you can do.
