Learn English – A verb for helping a criminal without knowing it

Perhaps you're referring to the idiom mind went blank.

Edit: I'm assuming you're referring to the sudden inability to recall information that you already know, in general. This could be someone's name (as in the example you provided), or it could be, say, the sudden inability to remember some factoid during an exam.

If I'm wrong about that assumption, however, there's this word:

lethonomia: 1. A tendency to forget names. 2. The inability to recall the right name.

The standard idiom for making decisions without important information is blindly, and it is frequently used in phrases such as "I am just following the crowd blindly" or "they just blindly followed orders."

Some caution might be in order, however. While this idiom is quite common, it could arguably be considered offensive to those who are actually physically blind.

(Compare also military jargon for transmitting a message "in the blind," meaning in the expectation that you can be heard, but without the ability to receive a message in return, as memorably used in the movie Gravity.)