I'm drawing a blank.
"I'm drawing a blank" means "I can't remember right now." You say this when you're trying to remember something that you used to know, but you can't remember it.
This is similar to the phrase "blank on ___":
"I'm drawing a blank" is fine to use in spoken English in pretty much any situation, but it's not often used in formal writing because you always have a chance to look up the information that you've forgotten!