lose (one's) train of thought
To be unable to resume one's previous progression of thought. A: "Therefore, it is critical to remember that—" B: "Professor! Will this be on the final exam?" A: "Yes, Stephen. Shoot, now I've lost my train of thought." And so you see, by collecting such data, we can make an inference to the, um… the… What am I trying to say? I lost my train of thought.
lose one's train of thought
Fig. to forget what one was talking or thinking about. Excuse me, I lost my train of thought. What was I talking about? Your question made the speaker lose her train of thought.