My ex-husband did not file our 2012 return. I was notified by the IRS. Can I file single for that year? I have no idea where he is to get his information.

If you were still filed as of 12/31/2012 you will need to file either as married filing jointly or married filing separately. Since you do not know where to get his information, you can file as married filing separately and print and mail the return in (do not forget about your state if you have to file a state return).