(one's) mind goes blank
One suddenly forgets or is unable to think of something. Even though I'd been preparing for the interview for days, my mind went blank as soon as they started asking questions. She went to introduce her husband to the producers, but her mind went blank when she tried to remember their names.
one's mind went blank
Fig. someone's mind has experienced total forgetfulness. He knew all his lines in rehearsal, but his mind went blank when he went before an audience.