Your poor DD, it's a terrible illness. Do you know if she has a diagnosis other than severe anxiety?
Don't want to hijack, but I was diagnosed with GAD (Generalised Anxiety Disorder) which means the anxiety isn't about one thing (like work or money, etc) but it can attach itself to whatever you end up focusing on. I also have had suicidal thoughts in the past (really bad last year). I'm 29, so quite close in age to your DD.
The first thing I'd say is there doesn't necessarily need to be a cause of the anxiety - sometimes a convergence of behaviour, circumstance and chemicals just kick it off. With the right medication and therapies, it can be controlled.
For lots of people, CBT (Cognitive Behavioural Therapy) is useful. I found it OK, but have responded much better to mindfulness-based therapies. A lot of people are finding mindfulness useful, and there are great books out there. I use this one: Mindfulness: Finding Peace in a Frantic World For me, mindfulness helps me be more at peace with this condition, which has the knock on effect of the anxiety being less strong and frequent. I also take medication.
Any therapy/meds are best discussed with health professionals, of course, but it's good to know there are a variety of resources out there. As far as going to see your DD goes, when I was ill I got a lot of strength from being around my parents and feeling safe with them. Maybe the best thing to do is keep in touch by phone or text at first, so she knows you're there, but ask her partner if he thinks she would benefit from you visiting. I'm so sure she would, but test the water with him. If she wants to speak about her feelings, let her. But if she doesn't bring them up, just be there with her - watch DVDs, curl up on the sofa.
Your DD sounds lovely - I'd place bets on her being a high achiever who asks a lot of herself and hates to disappoint anyone. That was me too. She can get better, but it's a process, one best done gently. It helps not to think of the situation in terms of sick/well, because that can breed thoughts of 'I'll never get better. This is it.' When that's not the case. It's shades of grey, a spectrum which she'll go up and down, and with the right help she will learn to live in a way that she feels comfortable with.