I completely blanked on an easy problem in a live interview. How do you recover mid-call?

It was basically a two-sum. Something I've done fifty times. The nerves just hit and my mind went white for what felt like a year. I mumbled through it and obviously didn't get the callback. What I can't figure out is what you actually do in the moment when the panic starts — do you say something out loud? Ask for a second to think? Genuinely asking, because I know it'll happen again.