I had that happen with me in college, I took on an ironing job for a fellow dorm-mate and messed it up. I do not know that you can salvage the situation; I think you have to tell her you are unable to make them for her, I would maybe say your machine is broken and you do not know when you are going to get it fixed or get back to being able to work on your sewing again, you're very sorry. Since she asked you to do the job (instead of you soliciting it?) it should not be quite as bad, she might be disappointed that you cannot make the curtains for her but it won't mean she hates you; it should be no worse than if you had said you did not have time or were not able to at the beginning.