Draw a blank
Definition 1 of 2
Pro Tip 1/3Memory Failure
Used when someone cannot recall information they're expected to know. 
When I tried to recall the capital of France, I drew a blank.
Pro Tip 2/3Temporary Lapse
Implies that the forgotten information might be recalled later. 
She drew a blank on the password now but remembered it the next day.
Pro Tip 3/3Embarrassing Moment
Often used in situations where forgetting something is awkward or uncomfortable. 
During introductions, I drew a blank on his name.