It will happen. At some point in an interview that matters, someone will ask you something and your mind will go completely blank, or you will simply not know. The panic that follows is usually worse than the situation, because candidates believe a single unanswered question sinks them. It almost never does. What sinks people is not the blank itself, but the flailing that comes after it.
Here is the thing worth internalising before your interview: how you handle not knowing is itself being assessed. In the real job, you will constantly face things you do not know. The interviewer is quietly interested in what you do at that edge, whether you bluff, freeze, and panic, or stay composed and handle it like an adult. A well-handled "I don't know" can actually score better than a mediocre bluff.
The first move: buy a moment, honestly
When the blank hits, the instinct is to start talking immediately to fill the silence. Resist it. The words that come out of a panic reflex are almost always worse than the ones that come from three seconds of thought.
It is completely acceptable to say, plainly, "That's a good question, let me think for a second." Then actually think. A short, deliberate pause reads as thoughtful and confident. A rushed, rambling non-answer reads as flustered. Interviewers are not unnerved by a few seconds of silence while someone considers a hard question. They are unnerved by someone visibly scrambling.
This pause alone solves a surprising number of "blanks", because often you do know, your brain just needs a moment to find it once the initial spike of panic passes.
If you genuinely do not know
Sometimes the pause does not rescue you and you simply do not have the answer. There are three good moves here, depending on the type of question.
For a knowledge question, be honest and show how you would find out. "I don't know that off the top of my head, but here is how I would go about working it out" is a strong answer, because it demonstrates the thing that actually matters in a job: not knowing everything, but knowing how to get to an answer. It is far better than a confident wrong answer, which is the worst outcome of all.
For a "tell me about a time" question where no example comes to mind, reach adjacent. If you cannot think of a perfect match, a close one is fine. "I don't have an example of exactly that, but a related situation was..." Interviewers would much rather have a slightly imperfect real example than watch you fabricate a perfect fake one. You can also ask for a moment: it is fine to say "let me think of the best example" and take a few seconds to search.
For a problem-solving or "how would you approach" question, think out loud. These are often designed to have no single right answer. The interviewer wants to see your reasoning, not a memorised solution. Talking through how you would break the problem down, what you would need to know, what the trade-offs are, is the answer. Silence and a request for the "right" answer misunderstands the question.
What never to do
Do not bluff confidently. A made-up answer delivered with certainty is the most damaging response available, because it suggests you will do the same thing on the job, assert something false rather than admit a gap. Interviewers are very good at spotting it, and a good follow-up question will expose it instantly.
Do not fall apart. One blank is one blank. If you let it rattle you, you carry the panic into the next three questions and turn a single stumble into a bad interview. Handle it, reset, move on. The interviewer will not be dwelling on it nearly as much as you are.
Do not over-apologise. "Oh gosh, I'm so sorry, I should know this, I can't believe I've gone blank" wastes time and amplifies a small moment into a visible crisis. A calm "I'm not sure on that one" is all it needs.
Do not ask to skip it and pretend it did not happen. If you can offer even a partial answer or your reasoning, do. Fully passing on a question leaves more of a mark than a composed, honest attempt.
The composure is the point
Notice that every good move here has the same quality: composure. The pause, the honest "here's how I'd find out", the thinking out loud, all of them are ways of staying level when you have been knocked slightly off balance. That composure is genuinely one of the things being tested, because it is exactly what the job will demand when a client asks you something you cannot immediately answer.
This is hard to practise by reading, because the skill is emotional, not intellectual. You need to have felt the blank and handled it, so the feeling is familiar rather than shocking. The way to build that is exposure: run enough mock interview questions, including hard and unexpected ones, that being asked something you did not prepare for stops feeling like an emergency and starts feeling normal. By the day, a blank becomes just another moment you know how to handle, rather than the thing you were dreading.