This happened to me recently. I've known this person for over six months know. I would consider us very close. Then, one day, gone. In my view, it was completely unexpected.
I can imagine that I am not alone. This has happened to you too, right?
So, what do you do? Do you give up? Do you keep trying to reach this friend? At what point does it turn into harassment? How often do you reach out to this friend?
I reached out once a week for three weeks. I would send random texts. I let her know that I was praying for, hoping she was having a good day, and that I missed her. Right when I was about to give up, I felt God tell me to push on. I sent one last text. I let her know that I was ready to stop trying at this 'friendship'. Finally, I received a response. We had a misunderstanding and now everything is all good.
Communication was key in this problem. She completely shut down because of a misunderstanding we had. If she had come to me four weeks prior, there never would have been any trouble.
Moral of the story here? Giving up is not an option. If you truly care about the person who seems to have fallen off of the face of the Earth, press on.
Your situation may be different than mine. Maybe the person needs some space. Give them a few weeks, and then reach out to them again. But, if you truly care about this person, never give up!
Just think... what if God gave up on you every time you ignored Him?
If God wouldn't do it to you, you shouldn't do it to someone!