I've shut down like that too before. I've also done the whole "ramble about 14 different things, none of which are what you really wanted to say" version of not saying much.

Whatever caused you to shut down, whatever fears, or self-protection, or "resistance," or whatever it is, it's part of what's going on with you and it's part of your relationship with your T. And now he's seen it happen, and that in itself is meaningful information even though it isn't what you wanted. It might be a good thing to discuss next time and could lead to new insights. So it's not really a waste, seen in that light; it's just a point along the way. But I know that's cold comfort when you really wanted to connect.