Bipolar Adult Daughter--please Help

My 20 year old adopted daughter, birthmother w/mental illness, has become a completely different person overnight. She is the oldest of 5 children and we have an incredibly close-knit family. She has overnight abandoned our entire family and has nothing to do with us, in spite of the fact that I was diagnosed with breast cancer just four months ago. She has gone from having incredibly high standards in regards to her morals and beliefs, to now living with a guy we have never met, failing all of her college classes, and completely disregarding any of her siblings loss and sadness as to her abandonment. She has been confronted by her younger siblings about the fact that she is not well, bipolar, and it simply makes her angry and resentful. How do we deal with this incredible loss...She has now announced to people via the internet that she is engaged to this person who we've never met and she has nothing to do with us. We have expressed our need for her in our lives, especially at such a time as this, but she simply doesn't care. Two hours after my mastectomy she waltzed into my post op room with a guy we met one other time and announced her engagement--broke up with him three days later and was living with this other guy...Please help...