hey there
i think "normal" calorie intake is 1,500 cals a day if you want to stay in shape
it all depends on your size and metabolism
but 850 is not enough to be healthy! your body burns a lot during the day, and if you excersize now you will need a lot more then that

try to stay calm there girl, i understand what you are going through.
just try to keep busy with friends and do whatever relaxes you. listen to good upbeat music, go for walks, get fresh air, join a group where you can meet new ppl, read a book, just stay positive!
its easier said than done i know, but maybe cut things out of magazines like i do and put them on your wall for motivation to stay positive.
make sure your getting lots of sleep rest too. take it one day at a time
best of wishes
*lucky*