Sorry to read this happened. It sucks when that happens, but more so when it's something deep and difficult that's been lost.

I'm afraid it's unlikely you'll get that back unless your browser supports or has enabled a feature whereby such text can be recovered by simply click the back button; this is a feature that has saved my butt a few times in Firefox, but I seem to recall it not working all the time.

A good practice is to copy the contents into the clipboard: select text, right-click, then select 'copy', or, depending on your setup, the Ctrl + C keyboard shortcut. You could also type out a long message into a text editor (such as Notepad, if you're on Windows) then copy and paste the text into PC when you're done.