lose one's mind

lose one's mind AND lose one's reason (go crazy, lose one's sanity) — сойти с ума, свихнуться, рехнуться, сбрендить (разг., прост.)

Example 1: Have you lost your mind? — Ты что, совсем рехнулся?

Example 2: I thought she'd lost her mind when she said she was going ice-fishing. (answers.com)

Example 3: That assignment is enough to make me lose my reason. (answers.com)

See also
[не все дома collection]
[lose it]
lose / be missing some of one's buttons / marbles (become / be mentally deficient)
have all one's buttons
have all one's marbles