For future reference: immediately pressing Cmd-z (or clicking Edit -> Undo) will undo the the damage. Given that it’s over 12 hours since you posted, I’m guessing that’s no longer an option.
I’ll let others more knowledgeable than I explain your options from here (I’m just another user). If you don’t get a reply here, try emailing technical support directly.
It was un utterly bizarre circumstance in that ‘undo’ wasn’t the first thing I thought of. I think I froze a bit as this was the first time I’ve ever lost such a body of text.
A long night has passed and I’ve re-written as best I can, but forced recollection is never as good as the original inspiration, so I’m still annoyed with myself.