If conversation lapses and I'm starting to panic, I try to... breathe. Relax. If still nothing comes to mind, I go for distraction: getting more water, asking for the desert menu, going to the bathroom. That can snap me out of it. And I sometimes prepare lists of questions I can ask if the going gets tough.
But even now I can fall into "I don't know what to say" mode, and it's hard to get out of. Too self-referential, probably. It's worst in groups.