Go to a doctor, go to a hospital, bring her to some sort of medical help. Is she on medication? Or in therapy? Medication and therapy combined is the best treatment for depression. She may fight you on going to seek medical help but it will ultimately save her life. I would do this ASAP. Just keep telling her you love her and everything will be alright.
j17c27 is right. Basically you need to do anything within reason that you can, but take decisive action. Get her a pet, get her up and moving and doing something. Go to a doctor, a therapist, a church, a friend - there isn't a magic right answer here (although in the immediate term, medication and physical affection from a loved one (person and/or pet) are pretty high up there - but she has to see that there are reasons to live and ultimately decide for herself that one or more of them are worth choosing to get up and try again for.
Most of us (especially on this forum) have actually been in the very same place. If it were my daughter, other than making sure I communicated I'll always love her no matter what, I'd try to get across that as much as she does not feel or believe this right now, these are feelings. As intense as they are, they do, by definition, change. She will feel better again. It is a when, not an if. And there are a lot of things she can do, and get help doing, that will dramatically increase the chances and speed of feeling better.
I know that this is probably the most harrowing thing a parent can go through (and speak of what I know), and I wish you the best.