When you added all of those comments, do you remember if you also clicked on the green "Send" button? If you didn't click on that button, the comments would never have been saved completely, and only existed in your browser's cache temporarily. If that cache gets cleared, the comments are lost.
It sounds like that might be what happened here, and as the comments never made it to Trello's servers, they won't be recoverable, unfortunately! I'm sorry!
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