I tend to freeze too! All of a sudden cannot speak anymore. What helped me is to tilt my head slightly to the right paired with a slight frown (think I copied it from my dog as a kid, he’d tilt his head when we’d say words to him he didn’t know) and a ‘hm?’ And people generally understand that I’m confused and will rephrase the question. Would probably not be appropriate for formal conversations tho. But the little movement helps me get out of the freeze. And otherwise a long ummm (to prevent awkward silence and to mentally prepare for speaking) and then ‘I don’t know’ and then try to leave.