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before you sent it ?
no there is not.
you could try searching your temp folders and temp internet folders for a phrase that you knew was in the writing. You might be able to find a file that had some of the missing text and open it in a text editor such as word pad and then copy it back to another email
but unless you specifically saved it as a draft there is no button that will pull it back up in gmail.
email is not a good format for writing complex long messages. You are much better off doing them in a word processor and then attaching or cut and paste into the email