Hi there and welcome
The only method would have been to use Control + Z which undoes the changes by replacing the text from the Ram in the computer. If this does nothing, then the text has gone from the Ram.
If you then closed the draft and re-opened it and still the text was gone, it means it updates the draft on the server.
This is different to actually deleting an email with which you have options such as in the Trash, or a deleted recovery process, as you overwrote the file, you did not technically delete it (it treats the entire file as a save, it does not save only the changed portions.
I am sorry to be the bearer of bad news, but in this situation it will not be possible sorry.
If you would like any additional information or assistance, please do not hesitate to let me know.
And if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.