be out of one's mind informal
lose one's mind informal

She looked up at me like I'd lost my mind.

They looked at him as if he'd lost his mind.

The guy looked at him like he had lost his mind.

He looked at me as if I had lost my mind.

She looked at him as if he had lost his mind.

He would have lost his mind if he'd gone back.

They looked at me as though I had lost my mind.

She wants to see me because she's lost her mind.

On the other hand, maybe she just lost her mind.

One of their party died along the way, another has lost his mind.