@AzulCrescent in this situation, you could also say the train of thought got derailed, which is a common joke when your attention got diverted and you can't remember what you were thinking about. :-)
@AzulCrescent the number of times I've had to wait my turn to talk only to forget what I was going to say is very large to the point where if I had a penny or for every time it happened in the last decade I would probably have enough to put a decent dent in this month's rent