When your mind goes blank it means you are experiencing a moment of forgetfulness. if your mind goes blank you are unable to answer a question or respond due to momentarily forgetting the correct answer. this is an informal colloquial idiom.

1. When I bumped into Jane outside the shop yesterday, it was so embarrassing because my mind went completely blank and I couldn't remember her name. I hope she didn't take offence! 
2. I am terrible with numbers if I need to do a sum in my head my mind just goes blank.

1. I spent a week preparing for my job interview, but when I got in there I was so nervous that I couldn't think of anything to say. My mind just went totally blank!
2. Sorry, my mind has gone blank, can you remind me what the sales numbers were for last month?