Is that happened? I'm not an expert on the history of Bretton Woods, but my understanding is countries that sent gold to be stored in the US retained ownership over it, and repatriation was never refused.
If they had converted gold to dollars, this only suspended their ability to "rebalance" between gold and dollars, and no value was lost.
The dollars were worth a small fraction of the gold. It's true that no value was lost — because that value was transferred from the other countries to the US.
That doesn't sound right. Money is fungible. Anyone who missed the "window" to convert in 1971 (which major countries like UK, Germany, and France didn't, as far as I can see) still had their dollars. The US-domiciled gold that backed the dollars had always belong the US; so the dollars being an "IOU" makes sense in a colloquial sense, I guess, but it's a gross simplification that misses the broader point about how Bretton Woods affected monetary policy. It shifted the US into being able to maintain a permanent trade deficit, and the US was free to devalue the dollar (effectively the world's default currency) without repercussion, which is good for the US but not for everyone else.
The dollars were worth much less than the gold. As soon as you couldn't exchange dollars for gold any more, the value of a dollar fell by a factor of several to reflect that.
If they had converted gold to dollars, this only suspended their ability to "rebalance" between gold and dollars, and no value was lost.