What if you can't think of a situation they ask about during a STAR interview?

I have 3 hours worth of STAR interview where the questions will be of the format "tell me about a situation where..."

Sometimes I might draw a blank or cannot come up with a response right away, in situations like this, do you fail you? I mean, when you have 3 years of work experience and so many things happened, it takes time to go over the 3 years and come up with a best scenario.

There might also be situations where you've just never experienced the situation they ask about...how would you deal with that?

Thanks

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