Hello & Welcome, Hoanglongmit.
I have seen this, experienced it, and heard about it happening in many places over time. Nevertheless, the pain you are feeling over your apparently lost friend is real, not matter how many other people have experienced it. It hurts, really hurts to be abandoned by someone close to you. I am sorry.
I do not wish to answer this question with a cliché, but it will have to do: "What should I do?" -- "The next thing." For a while as best you can keep doing the things you normally do, the things you have to do every day. Also, remember you are grieving a personal loss; allow yourself time to grieve. You cannot pretend the loss didn't happen or doesn't matter.

Is there perhaps someone at your school - teacher, nurse, counselor - or in your family with whom you could talk?