When your mind is a blank/goes blank, you cannot remember a particular thing, or you cannot remember anything.
His mind went blank in the exam.
Suddenly my mind went blank ( = I could not remember anything).
My heart began to race and my mind went blank.
I tried to remember her name, but my mind was a complete blank/went completely blank.
Her mind went blank.
I should know the answer, but my mind’s a blank.
This caught her off guard, then her mind went temporarily blank.