I typed a very important document in an email to retrieve later since I do not have a printer. I saved it in drafts but it did not save.I tried the back button. How do you get it back?

It sounds like you are using a web-based e-mail client. When using a web-based e-mail client, your e-mail is not saved on your computer. Instead, it is saved on to a server owned by the company that hosts your e-mail address. Since your computer does not save e-mails locally, your computer can not recover it. If the e-mail was not saved to your drafts, it can not be recovered.