You will have to file as married filing separately. Why won't your spouse give you her SS#? Something seems very suspicious. Are you ready for a divorce? Maybe counseling would help you both in your marriage and solve this minor tax issue.
If you're married, you can file as "married filing jointly" or "married filing separately" but you can't file "single". You may be able to obtain your spouses social security number by requesting a copy of your prior returns if you filed jointly in the past.
Evan A. Nielsen is licensed to practice law in California. The information provided here is for educational purposes only and is not intended as legal advice for a particular matter. This response does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult an attorney.
Based on your description I am going to assume that you either do not know the whereabouts of your spouse and/or your spouse will not cooperate with you in getting the taxes filed. Under those circumstances you should file as "married filing separately" - a joint return would be invalid because your spouse won't sign it, you will get in trouble if you forge his/her signature to the return, and since your spouse will not have known of the contents or agreed to the return, it would not count as his/her "return" for tax purposes.
In terms of your spouse's SSN, you obviously can't put it on the return, so you'll have to attach a disclosure statement to the return explaining why you can't put his/her SSN on the return. A disclosure statement like this can be made on Form 8275, Disclosure Statement, which you can download directly from the IRS website here: http://www.irs.gov/pub/irs-pdf/f8275.pdf
You will also have to be prepared to satisfactorily explain the situation and in particular what you've done to try and locate her/his SSN and why you couldn't obtain it. You have to be ready for this because the IRS will almost certainly (at least 90%) initially reject the return and request that you provide the missing information.
Given the situation you've described, I would strongly urge you to work with a competent local tax return preparer to get your return done properly; this is not something you should be attempting to do on your own. Also, do not simply rely on my answer - or anyone else's answer here - without also consulting with a competent local tax practitioner; you cannot just rely on answers you get for free online from people whom you've never met and whom you haven't retained to advise you.
My answer does not constitute legal advice and may not be relied upon by anyone for any purpose and does not constitute an attorney/client relationship or an offer to form such a relationship. This disclaimer is intended to be fully compliant with the requirements of Treasury Department Circular 230 and the terms thereof are fully incorporated by reference. If you wish to consult with me please contact me at dwatchley@newyorktaxcounsel or visit my website at www.newyorktaxcounsel.com