Consider these tips:
1.) THINK and PROCESS
First, give yourself time to think and process when it happens. It’s not that you have nothing to say, it’s more likely that you were either caught off guard or too nervous to concentrate. What helps is to either rephrase the question (while you think which one of your prepared examples you can state) or ask the interviewer for more details, such as “Can you elaborate on what exactly you mean by XYZ…?”
2.) SHARE your thoughts
Then, if it’s a question you don’t know how to answer, don’t be afraid to do a bit of thinking out loud. Many times hiring managers are asking tricky questions not to hear you present the right answer immediately, but to get a better understanding of how you think through problems.
3.) BE REAL (yet again)
And if you really don’t have any answer at all, it’s OK to be honest. A hiring manager will be more impressed that you’re willing to admit that than if you come up with a bad lie or go on and on about nothing really substantial. Plus, you can always say that you’ll get back to them on this question. This, in turn, gives you a great opportunity to elaborate on your “Thank you” follow up email afterwards.
Lastly: being nervous is absolutely normal – but the best tip of all to even fake confidence? Always come prepared so you never have to worry about this.