At first, have you asked your daughter if she wants to attend a boarding school? If you haven't, consider letting her change a school again. Choose one she would be able to attend without moving away from you. If she's so sad, it will make no good for her, and especially for her health. Boarding schooling is good, but at this age it isn't necessary.
Well OK, she may even realise her behaviour is making you sad, and stop crying, but... It wouldn't be about stopping being sad, just about hiding it, that may cause even more harm.
Basically, try just to have a sincere talk. Examine what she feels about that and what happens to her in that school. Does she miss her old friends as well? Has she experienced bullying there? Is there something good for her about studying at that school? Discuss pros and cons together, it may either solve your problem, or give you some clues about what to do. You may find out that you have to do just several little things to make her all right.
Also, what a child feels about a school is mainly what they does feel about its people. It's really hard to do anything, when you have no friends here, and your relatives are also away. There will be no good in her studies until she finds a couple of good friends, gets a favourite teacher, and so on. Other things are much less a problem than that.
All the things said above are taken fro my own experience. I'm a 20yo university student, and I had been sent to different boarding schools for a one-month summer try for multiple times (in Russia), but I finally made up my mind just before my final year simply because it was the first time ever when I met a couple of people it was really pleasant to communicate with.
I hope your daughter doesn't have such a communication disorder, but actually, the problem is all about this. Having friends to spend enough spare time with them will leave her almost no time for being sad. Be happy with it, and I wish you all being happy :)