One of your closest friends is suddenly turning into a different person: their typical level-headed behavior is gone, and suddenly they're off being wild and wacky. Why they are acting this way is a total mystery to you, but it might have to stay that way. Ask all the questions you want, but if they don't want to answer to you, they don't have to. Respect their boundaries and give them some space for now. They will come to you if they need your help.